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Filed Pursuant to Rule 424(b)(2)
Registration Statement No. 333-231146

The information in this preliminary prospectus supplement is not complete and may be changed. A registration statement relating to these securities has been filed with and declared effective by the Securities and Exchange Commission. This preliminary prospectus supplement and the accompanying prospectus are not an offer to sell and are not soliciting offers to buy these securities in any state where such offer or sale is not permitted.

SUBJECT TO COMPLETION, DATED JANUARY 12, 2021

PRELIMINARY PROSPECTUS SUPPLEMENT
(to Prospectus dated April 30, 2019)

$            

        % Notes due        

          We are offering $            in aggregate principal amount of % notes due            , which we refer to as the Notes. The Notes will mature on                    ,        . We will pay interest on the Notes on                and                of each year, beginning on                    , 2021.

          We may redeem the Notes in whole or in part at any time or from time to time, at the redemption price set forth under the caption "Description of the Notes—Optional Redemption" in this prospectus supplement. In addition, holders of the Notes can require us to repurchase some or all of the Notes at a purchase price equal to 100% of their principal amount, plus accrued and unpaid interest to, but not including, the repurchase date upon the occurrence of a "Change of Control Repurchase Event" (as defined herein). The Notes will be issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof.

          The Notes will be our direct unsecured obligations and rank pari passu with our existing and future unsecured indebtedness but will rank senior to our future indebtedness that is expressly subordinated in right of payment to the Notes issued by Main Street Capital Corporation. See "Summary of the Offering—Ranking of Notes."

          We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market ("LMM") companies and debt capital to middle market ("Middle Market") companies. Our LMM companies generally have annual revenues between $10 million and $150 million, and our LMM portfolio investments generally range in size from $5 million to $50 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies, with annual revenues typically between $150 million and $1.5 billion, and our Middle Market investments generally range in size from $3 million to $20 million.

          The LMM and Middle Market securities in which we invest generally would be rated below investment grade if they were rated by rating agencies. Below investment grade securities, which are often referred to as "junk," have predominantly speculative characteristics with respect to the issuer's capacity to pay interest and repay principal. They may also be difficult to value and are illiquid.

          Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity-related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company.

          We are an internally managed, closed-end, non-diversified management investment company that has elected to be treated as a business development company under the Investment Company Act of 1940, as amended.

          Investing in the Notes involves a high degree of risk and should be considered highly speculative. Before investing in the Notes, you should review carefully the risks and uncertainties, including the risk of leverage, described in the sections titled "Supplementary Risk Factors" beginning on page S-10 of this prospectus supplement, "Risk Factors" beginning on page 11 of the accompanying prospectus and in our most recently filed Annual Report on Form 10-K and subsequent filings with the Securities and Exchange Commission, or SEC, as well as under similar headings in the other documents that are filed on or after the date hereof and incorporated by reference into this prospectus supplement and the accompanying prospectus.

          This prospectus supplement and the accompanying prospectus contain important information about us that a prospective investor should know before investing in the Notes. We may also authorize one or more free writing prospectuses to be provided to you in connection with this offering. You should carefully read this prospectus supplement, the accompanying prospectus, and any related free writing prospectus, and any information incorporated by reference in each, before investing in the Notes and keep them for future reference. We file annual, quarterly and current reports, proxy statements and other information with the SEC. This information is available free of charge by contacting us at 1300 Post Oak Boulevard, 8th Floor, Houston, Texas 77056 or by telephone at (713) 350-6000 or on our website at www.mainstcapital.com. Information contained on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus, and you should not consider that information to be part of this prospectus supplement or the accompanying prospectus. The SEC also maintains a website at www.sec.gov that contains such information.

          Neither the Securities and Exchange Commission nor any state securities commission, nor any other regulatory body, has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

          The public offering price set forth above does not include accrued interest, if any. Interest on the Notes will accrue from January     , 2021 and must be paid by the purchaser if the Notes are delivered after January     , 2021.

          THE NOTES ARE NOT DEPOSITS OR OTHER OBLIGATIONS OF A BANK AND ARE NOT INSURED BY THE FEDERAL DEPOSIT INSURANCE CORPORATION OR ANY OTHER GOVERNMENT AGENCY.

          Delivery of the Notes in book-entry form only through The Depository Trust Company will be made on or about January     , 2021.

The date of this prospectus supplement is January     , 2021.

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ABOUT THE PROSPECTUS

        This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering of Notes and also adds to and updates information contained in the accompanying prospectus and the documents incorporated by reference into this prospectus supplement and the accompanying prospectus. The second part is the accompanying prospectus, which provides more information about us and related matters. To the extent the information contained in this prospectus supplement differs from the information contained in the accompanying prospectus or any document filed prior to the date of this prospectus supplement and incorporated herein by reference, the information in this prospectus supplement shall control. Generally, when we refer to this "prospectus," we are referring to both this prospectus supplement and the accompanying prospectus combined, together with any free writing prospectus that we have authorized for use in connection with this offering.

        You should rely only on the information included or incorporated by reference in this prospectus supplement and the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us that relates to this offering of Notes. Neither we nor the underwriters have authorized any other person to provide you with different information or to make representations as to matters not stated in this prospectus supplement, the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us that relates to this offering of Notes. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement, the accompanying prospectus and any free writing prospectus prepared by or on behalf of us that relates to this offering of Notes do not constitute an offer to sell, or a solicitation of an offer to buy, any Notes by any person in any jurisdiction where it is unlawful for that person to make such an offer or solicitation or to any person in any jurisdiction to whom it is unlawful to make such an offer or solicitation. You should not assume that the information included in this prospectus supplement, the accompanying prospectus or in any such free writing prospectus is accurate as of any date other than their respective dates, or that any information incorporated by reference herein or therein is accurate as of any date other than the date of the document incorporated by reference, regardless of the time of delivery of this prospectus supplement or sale of the Notes offered hereby.

Forward-Looking Statements

        Information included or incorporated by reference in this prospectus supplement, the accompanying prospectus and in any free writing prospectus relating to this offering of Notes may contain forward-looking statements, which can be identified by the use of forward-looking terminology such as "may," "will," "expect," "intend," "anticipate," "estimate," or "continue" or the negative thereof or other variations thereon or comparable terminology. The matters described in the sections titled "Supplementary Risk Factors" in this prospectus supplement, "Risk Factors" in the accompanying prospectus and "Risk Factors" in our most recent Annual Report on Form 10-K, as well as in subsequent filings with the SEC, or in any free writing prospectus relating to this offering and certain other factors noted throughout or incorporated by reference in this prospectus supplement, the accompanying prospectus and in any free writing prospectus relating to this offering constitute cautionary statements identifying important factors with respect to any such forward-looking statements, including certain risks and uncertainties that could cause actual results to differ materially from those in such forward-looking statements. We undertake no obligation to revise or update any forward-looking statements but advise you to consult any additional disclosures that we may make directly to you or through reports that we may file in the future with the SEC, including annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K.

        You should not place undue reliance on our forward-looking statements because the matters they describe are subject to known and unknown risks, uncertainties and other unpredictable factors, many

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of which are beyond our control. In addition to other information included or incorporated by reference in this prospectus supplement, please read carefully the sections titled "Risk Factors" and "Management's Discussion and Analysis of Financial Condition and Results of Operations" in our most recent Annual Report on Form 10-K, our most recent Quarterly Report on Form 10-Q and in other documents we may file with the SEC, as well as the section titled "Cautionary Statement Concerning Forward-Looking Statements" in the accompanying prospectus, before making an investment in the Notes offered hereby.

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SUPPLEMENTARY RISK FACTORS

        Investing in the Notes involves a high degree of risk. In addition to the other information contained in this prospectus supplement, the accompanying prospectus, the documents incorporated by reference, and any free writing prospectus that we may authorize for use in connection with this offering, you should carefully consider the following supplementary risk factors together with the risk factors set forth in the accompanying prospectus and as described in the section titled "Risk Factors" in our most recent Annual Report on Form 10-K, before making an investment in the Notes, as well as in subsequent filings with the SEC, including our most recent Quarterly Report on Form 10-Q, which are incorporated by reference into this prospectus supplement and the accompanying prospectus in their entirety. The risks described in these documents are not the only ones we face. Additional risks and uncertainties that we are unaware of, or that we currently believe are not material, may also become important factors that adversely affect our business. Past financial performance may not be a reliable indicator of future performance, and historical trends should not be used to anticipate results or trends in future periods. If any of these risks actually occur, our business, reputation, financial condition, results of operations, revenue, and future prospects could be seriously harmed. In such case, the market price, if any, of the Notes could decline, and you may lose part or all of your investment. Please also read carefully the section titled "Cautionary Statement Concerning Forward-Looking Statements" in the accompanying prospectus.

Risks Relating to Economic Conditions

Events outside of our control, including public health crises, could negatively affect our portfolio companies and our results of operations.

        Periods of market volatility have occurred and could continue to occur in response to pandemics or other events outside of our control. These types of events have adversely affected, and could continue to adversely affect, operating results for us and for our portfolio companies. For example, the COVID-19 pandemic has led to, and for an unknown period of time will continue to lead to, disruptions in local, regional, national and global markets and economies affected thereby, including the United States. With respect to U.S. and global credit markets and the economy in general, this outbreak has resulted in, and until fully resolved is likely to continue to result in, the following (among other things): (i) restrictions on travel and the temporary closure of many corporate offices, retail stores and manufacturing facilities and factories, resulting in significant disruption to the business of many companies, including supply chains and demand, as well as layoffs of employees; (ii) increased draws by borrowers on revolving lines of credit; (iii) increased requests by borrowers for amendments or waivers of their credit agreements to avoid default, increased defaults by borrowers and/or increased difficulty in obtaining refinancing; (iv) volatility in credit markets, including greater volatility in pricing and spreads; and (v) rapidly evolving proposals and actions by state and federal governments to address the problems being experienced by markets, businesses and the economy in general, which may not adequately address the problems being faced. The pandemic is having, and any future continuation of the pandemic could have, an adverse impact on the markets and the economy in general.

        Although it is impossible to predict the precise nature and consequences of these events, or of any political or policy decisions and regulatory changes occasioned by emerging events or uncertainty on applicable laws or regulations that impact us and our portfolio companies and investments, it is clear that these types of events are impacting and will, for at least some time, continue to impact us and our portfolio companies; in many instances the impact will be adverse and material. Any potential impact to our results of operations will depend to a large extent on future developments and new information that could emerge regarding the duration and severity of the COVID-19 pandemic and the actions taken by authorities and other entities to contain the spread or treat its impact, all of which are beyond our control. These potential impacts, while uncertain, could adversely affect our and our portfolio companies' operating results and financial condition.

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        The COVID-19 pandemic and the related disruption and financial distress experienced by our portfolio companies may have material adverse effects on our financial results, including interest income received from our investments and the underlying value of those investments. We may need to restructure our investments in certain portfolio companies as a result of the adverse effects of the COVID-19 pandemic, which could reduce the amount or extend the time for payment of principal or reduce the rate or extend the time of payment of interest, among other things. In addition, if an investment included in the borrowing base for the Credit Facility is deemed to have a material impairment or loss, or if we modify the terms of an investment included in the borrowing base for the Credit Facility, it may reduce the value of the borrowing base, which may have a material adverse effect on our available liquidity, results of operations and financial condition. In addition, any decreases in our net investment income would impact the portion of our cash flows dedicated to servicing existing borrowings under the Credit Facility, the 4.50% Notes and the 5.20% Notes and dividends to stockholders. Depending on the duration of the COVID-19 pandemic and the extent of its effects on our portfolio companies' operations and our operating results, any future dividends to our stockholders may be for amounts less than our historical dividends, may be made less frequently than historical practices and may also include return of capital.

        The 1940 Act generally prohibits us, as a BDC, from incurring indebtedness unless immediately after such borrowing we have an asset coverage, as defined in the 1940 Act, of at least 200% (or 150% if certain requirements are met). In addition, the Credit Facility and the indentures governing the 4.50% Notes and the 5.20% Notes contain similar limitations or covenants requiring our compliance with the 1940 Act asset coverage requirements, and the Credit Facility also contains other affirmative and negative covenants. A continued significant decrease in the value of our Investment Portfolio, resulting in significant further reductions of our net asset value as a result of the effects of the COVID-19 pandemic or otherwise increases the risk of us not meeting the required asset coverage requirement under the 1940 Act or breaching covenants under the Credit Facility or under the indentures governing the 4.50% Notes and the 5.20% Notes. Any such result could have a material adverse effect on our business, liquidity, financial condition, results of operations and ability to pay dividends to our stockholders and attributes thereof.

Risks Relating to the Notes

The Notes will be unsecured and therefore will be effectively subordinated to any current or future secured indebtedness, including indebtedness under the Credit Facility.

        The Notes will not be secured by any of our assets or any of the assets of our subsidiaries and will rank equally in right of payment with all of our existing and future unsubordinated, unsecured indebtedness. As a result, the Notes are effectively subordinated to any secured indebtedness we or our subsidiaries have currently incurred and may incur in the future (or any indebtedness that is initially unsecured to which we subsequently grant security) to the extent of the value of the assets securing such indebtedness. In any liquidation, dissolution, bankruptcy or other similar proceeding, the holders of any of our existing or future secured indebtedness and the secured indebtedness of our subsidiaries may assert rights against the assets pledged to secure that indebtedness in order to receive full payment of their indebtedness before the assets may be used to pay other creditors, including the holders of the Notes. As of January 11, 2021, we had $300.0 million outstanding under the Credit Facility out of $780.0 million in commitments. The indebtedness under the Credit Facility is senior to the Notes to the extent of the value of the assets securing such indebtedness.

The Notes will be structurally subordinated to the indebtedness and other liabilities of our subsidiaries.

        The Notes will be obligations exclusively of Main Street Capital Corporation and not of any of our subsidiaries. None of our subsidiaries is a guarantor of the Notes, and the Notes are not required to be guaranteed by any subsidiaries we may acquire or create in the future. In addition, several of our

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subsidiaries, specifically the Funds, maintain significant indebtedness and as a result the Notes are structurally subordinated to the indebtedness of these subsidiaries. For example, as of January 11, 2021, the Funds had collectively issued $309.8 million of the current regulatory maximum of $350.0 million of SBA-guaranteed debentures, which are included in our consolidated financial statements. The assets of such subsidiaries are not directly available to satisfy the claims of our creditors, including holders of the Notes. See "Management's Discussion and Analysis of Financial Condition and Results of Operations—Liquidity and Capital Resources" in our most recently filed Annual Report on Form 10-K, as well as in subsequent filings with the SEC, including our Quarterly Report on Form 10-Q for the quarter ended September 30, 2020, incorporated by reference herein for more detail on the SBA-guaranteed debentures.

        Except to the extent we are a creditor with recognized claims against our subsidiaries, all claims of other creditors of our subsidiaries will have priority over our equity interests in such subsidiaries (and therefore the claims of our creditors, including holders of the Notes) with respect to the assets of such subsidiaries. Even if we are recognized as a creditor of one or more of our subsidiaries, our claims would still be effectively subordinated to any security interests in the assets of any such subsidiary and to any indebtedness or other liabilities of any such subsidiary senior to our claims. Consequently, the Notes will be structurally subordinated to all indebtedness, including the SBA-guaranteed debentures, and other liabilities of any of our subsidiaries and any subsidiaries that we may in the future acquire or establish. In addition, our subsidiaries may incur substantial additional indebtedness in the future, all of which would be structurally senior to the Notes.

There is no active trading market for the Notes. If an active trading market does not develop for the Notes you may not be able to sell them.

        The Notes are a new issue of debt securities for which there currently is no trading market. We do not intend to list the Notes on any securities exchange or for quotation of the Notes on any automated dealer quotation system. If the Notes are traded after their initial issuance, they may trade at a discount to their initial offering price depending on prevailing interest rates, the market for similar securities, our credit ratings, our financial condition or other relevant factors. The underwriters may discontinue any market-making in the Notes at any time at their sole discretion. Accordingly, we cannot assure you that a liquid trading market will develop for the Notes, that you will be able to sell your Notes at a particular time or that the price you receive when you sell will be favorable. To the extent an active trading market does not develop, the liquidity and trading price for the Notes may be harmed. Accordingly, you may be required to bear the financial risk of an investment in the Notes for an indefinite period of time.

A downgrade, suspension or withdrawal of the credit rating assigned by a rating agency to us or the Notes, if any, or change in the debt markets could cause the liquidity or market value of the Notes to decline significantly.

        Our credit ratings are an assessment by rating agencies of our ability to pay our debts when due. Consequently, real or anticipated changes in our credit ratings will generally affect the market value of the Notes. These credit ratings may not reflect the potential impact of risks relating to the structure or marketing of the Notes. Credit ratings are not a recommendation to buy, sell or hold any security, and may be revised or withdrawn at any time by the issuing organization in its sole discretion. Neither we nor any underwriter undertakes any obligation to maintain our credit ratings or to advise holders of Notes of any changes in our credit ratings. The Notes will be rated by Standard & Poor's Ratings Services, or S&P. There can be no assurance that our credit ratings will remain for any given period of time or that such credit ratings will not be lowered or withdrawn entirely by the rating agency if in their judgment future circumstances relating to the basis of the credit ratings, such as adverse changes in our company, so warrant. In this regard, in March 2020, in connection with the onset of the

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COVID-19 pandemic, S&P downgraded our long-term issuer rating to "BBB–" with an "Outlook Stable." Further downgrades to us or our securities could increase our cost of capital or otherwise have a negative effect on our results of operations or financial condition. The conditions of the financial markets and prevailing interest rates have fluctuated in the past and are likely to fluctuate in the future, which could have an adverse effect on the market prices of the Notes.

The indenture under which the Notes will be issued contains limited protection for holders of the Notes.

        The indenture under which the Notes will be issued offers limited protection to holders of the Notes. The terms of the indenture and the Notes do not restrict our or any of our subsidiaries' ability to engage in, or otherwise be a party to, a variety of corporate transactions, circumstances or events that could have an adverse impact on your investment in the Notes. In particular, the terms of the indenture and the Notes will not place any restrictions on our or our subsidiaries' ability to:

•

issue securities or otherwise incur additional indebtedness or other obligations, including (1) any indebtedness or other obligations that would be equal in right of payment to the Notes, (2) any indebtedness or other obligations that would be secured and therefore rank effectively senior in right of payment to the Notes to the extent of the values of the assets securing such debt, (3) indebtedness of ours that is guaranteed by one or more of our subsidiaries and which therefore is structurally senior to the Notes and (4) securities, indebtedness or obligations issued or incurred by our subsidiaries that would be senior to our equity interests in our subsidiaries and therefore rank structurally senior to the Notes with respect to the assets of our subsidiaries, in each case other than an incurrence of indebtedness or other obligation that would cause a violation of Section 18(a)(1)(A) as modified by Section 61(a)(1) of the 1940 Act or any successor provisions, but giving effect, in each case, to any exemptive relief granted to us by the SEC (currently, this provision generally prohibits us from making additional borrowings, including through the issuance of additional debt or the sale of additional debt securities, unless our asset coverage, as defined in the 1940 Act, equals at least 200% (or 150% if certain requirements are met) after such borrowings);



•

pay dividends on, or purchase or redeem or make any payments in respect of, capital stock or other securities ranking junior in right of payment to the Notes, including subordinated indebtedness;



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sell assets (other than certain limited restrictions on our ability to consolidate, merge or sell all or substantially all of our assets);



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enter into transactions with affiliates;



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create liens (including liens on the shares of our subsidiaries) or enter into sale and leaseback transactions;



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make investments; or



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create restrictions on the payment of dividends or other amounts to us from our subsidiaries.

        Furthermore, the terms of the indenture and the Notes do not protect holders of the Notes in the event that we experience changes (including significant adverse changes) in our financial condition, results of operations or credit ratings, if any, as they do not require that we or our subsidiaries adhere to any financial tests or ratios or specified levels of net worth, revenues, income, cash flow or liquidity.

        Our ability to recapitalize, incur additional debt and take a number of other actions that are not limited by the terms of the Notes may have important consequences for you as a holder of the Notes, including making it more difficult for us to satisfy our obligations with respect to the Notes or negatively affecting the trading value of the Notes.

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        Other debt we issue or incur in the future could contain more protections for its holders than the indenture and the Notes, including additional covenants and events of default. The issuance or incurrence of any such debt with incremental protections could affect the market for and trading levels and prices of the Notes.

The optional redemption provision may materially adversely affect your return on the Notes.

        The Notes are redeemable in whole or in part upon certain conditions at any time or from time to time at our option. We may choose to redeem the Notes at times when prevailing interest rates are lower than the interest rate paid on the Notes. In this circumstance, you may not be able to reinvest the redemption proceeds in a comparable security at an effective interest rate as high as the Notes being redeemed.

We may not be able to repurchase the Notes upon a Change of Control Repurchase Event.

        We may not be able to repurchase the Notes upon a Change of Control Repurchase Event because we may not have sufficient funds. Upon a Change of Control Repurchase Event, holders of the Notes may require us to repurchase for cash some or all of the Notes at a repurchase price equal to 100% of the aggregate principal amount of the Notes being repurchased, plus accrued and unpaid interest to, but not including, the repurchase date. The terms of our Credit Facility provide that certain change of control events will constitute an event of default thereunder entitling the lenders to accelerate any indebtedness outstanding under our Credit Facility at that time and to terminate the Credit Facility. Our and our subsidiaries' future financing facilities may contain similar restrictions and provisions. Our failure to purchase such tendered Notes upon the occurrence of such Change of Control Repurchase Event would cause an event of default under the indenture governing the Notes and a cross-default under the agreements governing certain of our other indebtedness, which may result in the acceleration of such indebtedness requiring us to repay that indebtedness immediately. If a Change of Control Repurchase Event were to occur, we may not have sufficient funds to repay any such accelerated indebtedness. See "Description of the Notes—Offer to Repurchase Upon a Change of Control Repurchase Event" in this prospectus supplement for more information.

Our amount of debt outstanding will increase as a result of this offering, and if we default on our obligations to pay our other indebtedness, we may not be able to make payments on the Notes.

        As of January 11, 2021, we had approximately $1,244.8 million of principal indebtedness, including $300.0 million outstanding under the Credit Facility, $309.8 million outstanding from SBA-guaranteed debentures, $185.0 million of the 4.50% Notes and $450.0 million of the 5.20% Notes outstanding. Any default under the agreements governing our indebtedness, including a default under the Credit Facility, the 4.50% Notes, the 5.20% Notes or under other indebtedness to which we may be a party that is not waived by the required lenders or debt holders, and the remedies sought by the holders of such indebtedness could make us unable to pay principal, premium, if any, and interest on the Notes and substantially decrease the market value of the Notes. If we are unable to generate sufficient cash flow and are otherwise unable to obtain funds necessary to meet required payments of principal, premium, if any, and interest on our indebtedness, or if we otherwise fail to comply with the various covenants, including financial and operating covenants, in the instruments governing our indebtedness, we could be in default under the terms of the agreements governing such indebtedness. In the event of such default, the holders of such indebtedness could elect to declare all the funds borrowed thereunder to be due and payable, together with accrued and unpaid interest, the lenders under the Credit Facility or other debt we may incur in the future could elect to terminate their commitments, cease making further loans and institute foreclosure proceedings against our assets, and we could be forced into bankruptcy or liquidation. Our ability to generate sufficient cash flow in the future is, to some extent, subject to general economic, financial, competitive, legislative and regulatory factors as well as other

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factors that are beyond our control. We cannot assure you that our business will generate cash flow from operations, or that future borrowings will be available to us under the Credit Facility or otherwise, in an amount sufficient to enable us to meet our payment obligations under the Notes and our other debt and to fund other liquidity needs.

        If our operating performance declines and we are not able to generate sufficient cash flow to service our debt obligations, we may in the future need to refinance or restructure our debt, including the Notes, sell assets, reduce or delay capital investments, seek to raise additional capital or seek to obtain waivers from the required lenders under the Credit Facility or the required holders of the 4.50% Notes, the 5.20% Notes or other debt that we may incur in the future to avoid being in default. If we are unable to implement one or more of these alternatives, we may not be able to meet our payment obligations under the Notes and our other debt. If we breach our covenants under the Credit Facility, the 4.50% Notes, the 5.20% Notes or other debt and seek a waiver, we may not be able to obtain a waiver from the required lenders or debt holders. If this occurs, we would be in default under the Credit Facility, the 4.50% Notes, the 5.20% Notes or other debt, the lenders or debt holders could exercise their rights as described above, and we could be forced into bankruptcy or liquidation. If we are unable to repay debt, lenders having secured obligations could proceed against the collateral securing the debt. Because the Credit Facility has, and any future credit facilities will likely have, customary cross-default provisions, if the indebtedness under the Notes, the 4.50% Notes, the 5.20% Notes, the Credit Facility or under any future credit facility is accelerated, we may be unable to repay or finance the amounts due.

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USE OF PROCEEDS

        We estimate that the net proceeds we will receive from the sale of the $             million aggregate principal amount of the Notes in this offering will be approximately $             million, based on a public offering price of        % of par, after deducting the underwriting discount and estimated offering expenses payable by us.

        We intend to initially use the net proceeds from this offering to repay outstanding debt borrowed under our Credit Facility. However, through re-borrowing of the initial repayments under our Credit Facility, we intend to use the net proceeds from this offering to make investments in accordance with our investment objective and strategies described in this prospectus supplement and the accompanying prospectus, to make investments in marketable securities and idle funds investments, which may include investments in secured intermediate term bank debt, rated debt securities and other income producing investments, to pay our operating expenses and other cash obligations, and for general corporate purposes. Our ability to achieve our investment objective may be limited to the extent that the net proceeds from an offering, pending full investment, are held in interest bearing deposits or other short-term instruments. See "Risk Factors—Risks Relating to Our Securities—We may be unable to invest a significant portion of the net proceeds from an offering or from exiting an investment or other capital on acceptable terms, which could harm our financial condition and operating results" in our most recently filed Annual Report on Form 10-K, as well as in subsequent filings with the SEC, incorporated by reference herein.

        On January 11, 2021, we had approximately $300.0 million outstanding under our Credit Facility. Our Credit Facility matures in September 2023, unless extended, and bears interest, at our election, on a per annum basis at a rate equal to the applicable LIBOR rate plus (A) 1.875% (or the applicable base rate plus 0.875%) so long as we meet certain agreed upon excess collateral and maximum leverage requirements or (B) 2.0% (or the applicable base rate plus 1.0%) otherwise. Amounts repaid under our Credit Facility will remain available for future borrowings.

        Affiliates of RBC Capital Markets, LLC, Truist Securities, Inc., SMBC Nikko Securities America, Inc., and Raymond James & Associates, Inc., underwriters in this offering, act as lenders and/or agents under our Credit Facility. As described above, we intend to use net proceeds of this offering to repay the outstanding indebtedness under our Credit Facility, and such affiliates therefore may receive a portion of the proceeds from this offering through the repayment of those borrowings. See "Underwriting (Conflicts of Interest)—Conflicts of Interest" below.

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CAPITALIZATION

        The following table sets forth our capitalization:

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on an actual basis as of September 30, 2020; and



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on an as-adjusted basis giving effect to the sale of $             million aggregate principal amount of Notes in this offering, less estimated underwriting discounts and offering expenses payable by us, and the application of the proceeds thereof.

        This table should be read in conjunction with "Management's Discussion and Analysis of Financial Condition and Results of Operations" and consolidated financial statements and the related notes thereto in our most recently filed Annual Report on Form 10-K, as well as in subsequent filings with the SEC, including our Quarterly Report on Form 10-Q for the quarter ended September 30, 2020, incorporated by reference herein.

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DESCRIPTION OF THE NOTES

        The following description of the particular terms of the Notes supplements and, to the extent inconsistent with, replaces the description of the general terms and provisions of the debt securities set forth in the accompanying prospectus.

        We will issue the Notes under a base indenture dated as of April 2, 2013, between us and The Bank of New York Mellon Trust Company, N.A., as trustee, or "the trustee," as supplemented by a separate supplemental indenture, to be dated as of the settlement date for the Notes. As used in this section, all references to the "indenture" mean the base indenture as supplemented by the supplemental indenture. The terms of the Notes include those expressly set forth in the indenture and those made part of the indenture by reference to the Trust Indenture Act of 1939, as amended.

        The following description is a summary of the material provisions of the Notes and the indenture and does not purport to be complete. This summary is subject to and is qualified by reference to all the provisions of the Notes and the indenture, including the definitions of certain terms used in the indenture. We urge you to read these documents because they, and not this description, define your rights as a holder of the Notes. You may request a copy of the indenture from us by making a written request to Main Street Capital Corporation, 1300 Post Oak Boulevard, 8th Floor, Houston, TX 77056, or by calling us collect at (713) 350-6000, or by visiting our website at www.mainstcapital.com. The information on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus.

        For purposes of this description, references to "we," "our" and "us" refer only to Main Street Capital Corporation and not to any of our current or future subsidiaries and references to "subsidiaries" refer only to our consolidated subsidiaries and exclude any investments held by Main Street Capital Corporation in the ordinary course of business which are not, under GAAP, consolidated on the financial statements of Main Street Capital Corporation and its subsidiaries.

General

        The Notes:

•

will be our general unsecured, senior obligations;



•

will initially be issued in an aggregate principal amount of $            ;



•

will mature on                ,                 , unless earlier redeemed or repurchased, as discussed below;



•

will bear cash interest from January     , 2021 at an annual rate of        % payable semiannually on                 and                of each year, beginning on                        ,2021;



•

will be subject to redemption at our option as described under "—Optional Redemption";



•

will be subject to repurchase by us at the option of the holders following a Change of Control Repurchase Event (as defined below under "—Offer to Repurchase Upon a Change of Control Repurchase Event"), at a repurchase price equal to 100% of the principal amount of the Notes to be repurchased, plus accrued and unpaid interest to, but excluding, the date of repurchase;



•

will be issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof; and



•

will be represented by one or more registered Notes in global form, but in certain limited circumstances may be represented by Notes in definitive form. See "—Book-Entry, Settlement and Clearance."

        The indenture does not limit the amount of debt that may be issued by us or our subsidiaries under the indenture or otherwise, but does contain a covenant regarding our asset coverage that would

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have to be satisfied at the time of our incurrence of additional indebtedness. See "—Covenants—Other Covenants." The indenture does not contain any financial covenants and does not restrict us from paying dividends or distributions or issuing or repurchasing our other securities. Other than restrictions described under "—Offer to Repurchase Upon a Change of Control Repurchase Event" and "—Merger, Consolidation or Sale of Assets" below, the indenture does not contain any covenants or other provisions designed to afford holders of the Notes protection in the event of a highly leveraged transaction involving us or in the event of a decline in our credit rating as the result of a takeover, recapitalization, highly leveraged transaction or similar restructuring involving us that could adversely affect such holders.

        We may, without the consent of the holders, issue additional Notes under the indenture with the same terms as the Notes offered hereby in an unlimited aggregate principal amount; provided that, if such additional Notes are not fungible with the Notes offered hereby (or any other tranche of additional Notes) for U.S. federal income tax purposes, then such additional Notes will have different CUSIP numbers from the Notes offered hereby (and any such other tranche of additional Notes).

        We do not intend to list the Notes on any securities exchange or any automated dealer quotation system.

Payments on the Notes; Paying Agent and Registrar; Transfer and Exchange

        We will pay the principal of, and interest on, the Notes in global form registered in the name of or held by DTC, or its nominee in immediately available funds to DTC or its nominee, as the case may be, as the registered holder of such Global Note (as defined below).

        Payment of principal of (and premium, if any) and any such interest on the Notes will be made at the corporate trust office of the trustee in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; provided, however, that at our option payment of interest may be made by check mailed to the address of the person entitled thereto as such address shall appear in the security register.

        A holder of the Notes may transfer or exchange Notes at the office of the security registrar in accordance with the indenture. The security registrar and the trustee may require a holder, among other things, to furnish appropriate endorsements and transfer documents. No service charge will be imposed by us, the trustee or the security registrar for any registration of transfer or exchange of Notes, but we may require a holder to pay a sum sufficient to cover any transfer tax or other similar governmental charge required by law or permitted by the indenture.

        The registered holder of a Note will be treated as its owner for all purposes.

Interest

        The Notes will bear cash interest at a rate of        % per year until maturity. Interest on the Notes will accrue from January     , 2021 or from the most recent date on which interest has been paid or duly provided for. Interest will be payable semiannually in arrears on                and                 of each year, beginning on                        ,2021.

        Interest will be paid to the person in whose name a Note is registered at 5:00 p.m. New York City time, or the close of business, on                or                , as the case may be, immediately preceding the relevant interest payment date, or each, a "regular record date." Interest on the Notes will be computed on the basis of a 360-day year composed of twelve 30-day months.

        If any interest payment date, the maturity date or any earlier required repurchase date upon a Change of Control Repurchase Event (defined below) of a Note falls on a day that is not a business day, the required payment will be made on the next succeeding business day and no interest on such

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payment will accrue in respect of the delay. The term "business day" means, with respect to any Note, each Monday, Tuesday, Wednesday, Thursday, and Friday that is not a day on which banking institutions in The City of New York are authorized or obligated by law or executive order to close.

Ranking

        The Notes will be our general unsecured obligations that rank pari passu with our existing and future general unsecured and senior unsecured indebtedness, including the 4.50% Notes and the 5.20% Notes. The Notes will rank senior to any of our future indebtedness that expressly states it is subordinated to the Notes and effectively subordinated to all of our existing and future secured indebtedness (including indebtedness that is initially unsecured, but to which we subsequently grant security), to the extent of the value of the assets securing such indebtedness, including approximately $300.0 million of borrowings outstanding as of January 11, 2021 under our Credit Facility to the extent of the value of the assets securing the Credit Facility. The Notes will rank structurally subordinated to all existing and future indebtedness and other obligations of any of our subsidiaries, financing vehicles or similar facilities, including the Funds' $309.8 million of SBIC debentures outstanding as of January 11, 2021. In the event of our bankruptcy, liquidation, reorganization or other winding up, our assets that secure secured debt will be available to pay obligations on the Notes only after all indebtedness under such secured debt has been repaid in full from such assets. We advise you that there may not be sufficient assets remaining to pay amounts due on any or all the Notes then outstanding.

Optional Redemption

        We may redeem some or all of the Notes at any time, or from time to time. If we choose to redeem any Notes prior to maturity, we will pay a redemption price (as determined by us) equal to the greater of the following amounts, plus, in each case, accrued and unpaid interest to, but excluding, the redemption date:

•

100% of the principal amount of the Notes to be redeemed, or



•

the sum of the present values of the remaining scheduled payments of principal and interest (exclusive of accrued and unpaid interest to the date of redemption) on the Notes to be redeemed, discounted to the redemption date on a semiannual basis (assuming a 360-day year consisting of twelve 30-day months) using the applicable Treasury Rate plus            basis points.

        If we choose to redeem any Notes, we will deliver a notice of redemption to holders of the Notes not less than 30 nor more than 60 days before the redemption date. If we are redeeming less than all of the Notes, the particular Notes to be redeemed will be selected in accordance with the indenture and, so long as the Notes are registered to DTC or its nominee, in accordance with the procedures of DTC; provided, however, that no such partial redemption shall reduce the portion of the principal amount of a Note not redeemed to less than $2,000. Unless we default in payment of the redemption price, on and after the redemption date, interest will cease to accrue on the Notes or portions of the Notes called for redemption.

        For purposes of calculating the redemption price in connection with the redemption of the Notes, on any redemption date, the following terms have the meanings set forth below:

        "Treasury Rate" means, with respect to any redemption date, the rate per annum equal to the semiannual equivalent yield-to-maturity of the Comparable Treasury Issue (computed as of the third business day immediately preceding the redemption), assuming a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for such redemption date. The redemption price and the Treasury Rate will be determined by us.

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        "Comparable Treasury Issue" means the United States Treasury security selected by the Reference Treasury Dealer as having a maturity comparable to the remaining term of the Notes to be redeemed that would be utilized, at the time of selection and in accordance with customary financing practice, in pricing new issues of corporate debt securities of comparable maturity to the remaining term of the Notes being redeemed.

        "Comparable Treasury Price" means (1) the average of the remaining Reference Treasury Dealer Quotations for the redemption date, after excluding the highest and lowest Reference Treasury Dealer Quotations, or (2) if the Quotation Agent obtains fewer than four such reference treasury dealer quotations, the average of all such quotations.

        "Quotation Agent" means a Reference Treasury Dealer selected by us.

        "Reference Treasury Dealer" means RBC Capital Markets, LLC and four other financial institutions selected by us, or their affiliates, which are primary U.S. government securities dealers and their respective successors; provided, however, that if any of the foregoing or their affiliates shall cease to be a primary U.S. government securities dealer in the United States, or a "Primary Treasury Dealer," we shall select another Primary Treasury Dealer.

        "Reference Treasury Dealer Quotations" means, with respect to each Reference Treasury Dealer and any redemption date, the average, as determined by the Quotation Agent, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Quotation Agent by such Reference Treasury Dealer at 3:30 p.m. New York City time on the third business day preceding such redemption date.

        All determinations made by any Reference Treasury Dealer, including the Quotation Agent, with respect to determining the redemption price will be final and binding absent manifest error.

        In addition, we may at any time purchase any of the Notes in the open market or in private transactions, at differing prices, or by tender, subject to applicable law.

Offer to Repurchase Upon a Change of Control Repurchase Event

        If a Change of Control Repurchase Event occurs, unless we have exercised our right to redeem the Notes in full, we will make an offer to each holder of Notes to repurchase all or any part (in minimum denominations of $2,000 and integral multiples of $1,000 principal amount) of that holder's Notes at a repurchase price in cash equal to 100% of the aggregate principal amount of Notes repurchased plus any accrued and unpaid interest on the Notes repurchased to, but excluding, the date of repurchase. Within 30 days following any Change of Control Repurchase Event or, at our option, prior to any Change of Control, but after the public announcement of the Change of Control, we will mail a notice to each holder describing the transaction or transactions that constitute or may constitute the Change of Control Repurchase Event and offering to repurchase Notes on the payment date specified in the notice, which date will be no earlier than 30 days and no later than 60 days from the date such notice is mailed. The notice shall, if mailed prior to the date of consummation of the Change of Control, state that the offer to purchase is conditioned on the Change of Control Repurchase Event occurring on or prior to the payment date specified in the notice. We will comply with the requirements of Rule 14e-1 promulgated under the Exchange Act and any other securities laws and regulations thereunder to the extent those laws and regulations are applicable in connection with the repurchase of the Notes as a result of a Change of Control Repurchase Event. To the extent that the provisions of any securities laws or regulations conflict with the Change of Control Repurchase Event provisions of the Notes, we will comply with the applicable securities laws and regulations and will not be deemed to have breached our obligations under the Change of Control Repurchase Event provisions of the Notes by virtue of such conflict.

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        On the Change of Control Repurchase Event payment date, subject to extension if necessary to comply with the provisions of the 1940 Act and the rules and regulations promulgated thereunder, we will, to the extent lawful:

•

accept for payment all Notes or portions of Notes properly tendered pursuant to our offer;



•

deposit with the paying agent an amount equal to the aggregate purchase price in respect of all Notes or portions of Notes properly tendered; and



•

deliver or cause to be delivered to the trustee the Notes properly accepted, together with an officers' certificate stating the aggregate principal amount of Notes being purchased by us.

        The paying agent will promptly remit to each holder of Notes properly tendered the purchase price for the Notes, and the trustee will promptly authenticate and mail (or cause to be transferred by book-entry) to each holder a new Note equal in principal amount to any unpurchased portion of any Notes surrendered; provided that each new Note will be in a minimum principal amount of $2,000 or an integral multiple of $1,000 in excess thereof.

        We will not be required to make an offer to repurchase the Notes upon a Change of Control Repurchase Event if a third party makes an offer in the manner, at the times and otherwise in compliance with the requirements for an offer made by us and such third party purchases all Notes properly tendered and not withdrawn under its offer.

        The source of funds that will be required to repurchase Notes in the event of a Change of Control Repurchase Event will be our available cash or cash generated from our operations or other potential sources, including funds provided by a purchaser in the Change of Control transaction, borrowings, sales of assets or sales of equity. We cannot assure you that sufficient funds from such sources will be available at the time of any Change of Control Repurchase Event to make required repurchases of Notes tendered. The terms of our Credit Facility provide that certain change of control events will constitute an event of default thereunder entitling the lenders to accelerate any indebtedness outstanding under our Credit Facility at that time and to terminate the Credit Facility. Our and our subsidiaries' future financing facilities may contain similar restrictions and provisions. It is possible that we will not have sufficient funds at the time of the Change of Control Repurchase Event to make the required repurchase of the Notes and/or our and our subsidiaries' other debt. See "Risk Factors—We may not be able to repurchase the Notes upon a Change of Control Repurchase Event" in this prospectus supplement.

        The definition of "Change of Control" includes a phrase relating to the direct or indirect sale, lease, transfer, conveyance or other disposition of "all or substantially all" of our assets and those of our Controlled Subsidiaries taken as a whole. Although there is a limited body of case law interpreting the phrase "substantially all," there is no precise, established definition of the phrase under applicable law. Accordingly, the ability of a holder of Notes to require us to repurchase the Notes as a result of a sale, lease, transfer, conveyance or other disposition of less than all of our assets and the assets of our Controlled Subsidiaries taken as a whole to another person or group may be uncertain.

        For purposes of the Notes:

        "Below Investment Grade Rating Event" means the Notes are downgraded below Investment Grade by the Rating Agency on any date from the date of the public notice of an arrangement that results in a Change of Control until the end of the 60-day period following public notice of the occurrence of a Change of Control (which period shall be extended so long as the rating of the Notes is under publicly announced consideration for possible downgrade by the Rating Agency); provided that a Below Investment Grade Rating Event otherwise arising by virtue of a particular reduction in rating shall not be deemed to have occurred in respect of a particular Change of Control (and thus shall not be deemed a Below Investment Grade Rating Event for purposes of the definition of Change of

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Control Repurchase Event hereunder) if the Rating Agency making the reduction in rating to which this definition would otherwise apply does not announce or publicly confirm or inform the trustee in writing at our request that the reduction was the result, in whole or in part, of any event or circumstance comprised of or arising as a result of, or in respect of, the applicable Change of Control (whether or not the applicable Change of Control shall have occurred at the time of the Below Investment Grade Rating Event).

        "Change of Control" means the occurrence of any of the following:

•

the direct or indirect sale, lease, transfer, conveyance or other disposition (other than by way of merger or consolidation) in one or a series of related transactions, of all or substantially all of the assets of Main Street Capital Corporation and its Controlled Subsidiaries taken as a whole to any "person" or "group" (as those terms are used in Section 13(d)(3) of the Exchange Act), other than to any Permitted Holders; provided that, for the avoidance of doubt, a pledge of assets pursuant to any secured debt instrument of Main Street Capital Corporation or its Controlled Subsidiaries shall not be deemed to be any such sale, lease, transfer, conveyance or disposition;



•

the consummation of any transaction (including, without limitation, any merger or consolidation) the result of which is that any "person" or "group" (as those terms are used in Section 13(d)(3) of the Exchange Act) (other than any Permitted Holders) becomes the "beneficial owner" (as defined in Rules 13d-3 and 13d-5 promulgated under the Exchange Act), directly or indirectly, of more than 50% of the outstanding Voting Stock of Main Street Capital Corporation, measured by voting power rather than number of shares; or



•

the approval by Main Street Capital Corporation's stockholders of any plan or proposal relating to the liquidation or dissolution of Main Street Capital Corporation.

        "Change of Control Repurchase Event" means the occurrence of a Change of Control and a Below Investment Grade Rating Event.

        "Controlled Subsidiary" means any subsidiary of Main Street Capital Corporation, 50% or more of the outstanding equity interests of which are owned by Main Street Capital Corporation and its direct or indirect subsidiaries and of which Main Street Capital Corporation possesses, directly or indirectly, the power to direct or cause the direction of the management or policies, whether through the ownership of voting equity interests, by agreement or otherwise.

        "Investment Grade" means a rating of BBB– or better by S&P (or its equivalent under any successor rating categories of S&P) (or, if such Rating Agency ceases to rate the Notes for reasons outside of our control, the equivalent investment grade credit rating from any Rating Agency selected by us as a replacement Rating Agency).

        "Permitted Holders" means (i) us and (ii) one or more of our Controlled Subsidiaries.

        "Rating Agency" means:

•

S&P; and



•

if S&P ceases to rate the Notes or fails to make a rating of the Notes publicly available for reasons outside of our control, a "nationally recognized statistical rating organization" as defined in Section (3)(a)(62) of the Exchange Act selected by us as a replacement agency for S&P.

        "S&P" means Standard & Poor's Ratings Services, a division of McGraw-Hill, Inc., or any successor thereto.

        "Voting Stock" as applied to stock of any person, means shares, interests, participations or other equivalents in the equity interest (however designated) in such person having ordinary voting power for

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the election of a majority of the directors (or the equivalent) of such person, other than shares, interests, participations or other equivalents having such power only by reason of the occurrence of a contingency.

Covenants

        In addition to the covenants described in the base indenture, the following covenants shall apply to the Notes. To the extent of any conflict or inconsistency between the base indenture and the following covenants, the following covenants shall govern:

Merger, Consolidation or Sale of Assets

        Under the terms of the indenture, we are generally permitted to consolidate or merge with another entity. We are also permitted to sell all or substantially all of our assets to another entity. However, we may not take any of these actions unless all the following conditions are met:

•

we are the continuing entity or, if we merge out of existence or sell our assets, the resulting or transferee entity must agree to be legally responsible for our obligations under the Notes;



•

the merger or sale of assets must not cause a default on the Notes and we must not already be in default (unless the merger or sale would cure the default). For purposes of this no-default test, a default would include an Event of Default that has occurred and has not been cured, as described under "Events of Default" below. A default for this purpose would also include any event that would be an Event of Default if the requirements for giving us a notice of default or our default having to exist for a specific period of time were disregarded;



•

we must deliver certain certificates and documents to the trustee; and



•

we must satisfy any other requirements specified in the indenture.

        Although there is a limited body of case law interpreting the phrase "substantially all," there is no precise established definition of the phrase under applicable law. Accordingly, in certain circumstances there may be a degree of uncertainty as to whether a particular transaction would involve "all or substantially all" of the properties or assets of a person. As a result, it may be unclear as to whether the merger, consolidation or sale of assets covenant would apply to a particular transaction as described above absent a decision by a court of competent jurisdiction. Although these types of transactions are permitted under the indenture, certain of the foregoing transactions could constitute a Change of Control that results in a Change of Control Repurchase Event permitting each holder to require us to repurchase the Notes of such holder as described above.

        An assumption by any person of obligations under the Notes and the indenture might be deemed for U.S. federal income tax purposes to be an exchange of the Notes for new Notes by the holders thereof, resulting in recognition of gain or loss for such purposes and possibly other adverse tax consequences to the holders. Holders should consult their own tax advisors regarding the tax consequences of such an assumption.

Other Covenants

•

We agree that for the period of time during which the Notes are outstanding, we will not violate Section 18(a)(1)(A) as modified by Section 61(a)(1) of the 1940 Act or any successor provisions, whether or not we continue to be subject to such provisions of the 1940 Act, but giving effect, in either case, to any exemptive relief granted to us by the SEC. Currently, these provisions generally prohibit us from making additional borrowings, including through the issuance of additional debt or the sale of additional debt securities, unless our asset coverage, as defined in the 1940 Act, equals at least 200% (or 150% if certain requirements are met) after such

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borrowings. See "Risk Factors—Risks Relating to Our Business and Structure—Previously enacted legislation may allow us to incur additional leverage" in our most recently filed Annual Report on Form 10-K, as well as in subsequent filings with the SEC, incorporated by reference herein.

•

If, at any time, we are not subject to the reporting requirements of Sections 13 or 15(d) of the Exchange Act to file any periodic reports with the Commission, we agree to furnish to holders of the Notes and the trustee, for the period of time during which the Notes are outstanding, our audited annual consolidated financial statements, within 90 days of our fiscal year end, and unaudited interim consolidated financial statements, within 45 days of our fiscal quarter end (other than our fourth fiscal quarter). All such financial statements will be prepared, in all material respects, in accordance with GAAP, as applicable.

Events of Default

        The term "Event of Default" in respect of the Notes means any of the following:

•

default in the payment of interest upon any Note when due and payable, and continuance of such default for a period of 30 days;



•

default in the payment of the principal (or premium, if any) of any Note when it becomes due and payable at its maturity, including upon any redemption date or required repurchase date;



•

we remain in breach of any other covenant or agreement in respect of the Notes for 60 days after we receive a written notice of default stating we are in breach. The notice must be sent by either the trustee or holders of at least 25% of the principal amount of the outstanding Notes;



•

default by us or any of our significant subsidiaries, as defined in Article 1, Rule 1-02 of Regulation S-X promulgated under the Exchange Act (but excluding any subsidiary which is (a) a non-recourse or limited recourse subsidiary, (b) a bankruptcy remote special purpose vehicle, or (c) is not consolidated with Main Street Capital Corporation for purposes of GAAP), with respect to any mortgage, agreement or other instrument under which there may be outstanding, or by which there may be secured or evidenced, any indebtedness for money borrowed in excess of $50 million in the aggregate of us and/or any such subsidiary, whether such indebtedness now exists or shall hereafter be created (i) resulting in such indebtedness becoming or being declared due and payable or (ii) constituting a failure to pay the principal or interest of any such debt when due and payable at its stated maturity, upon required repurchase, upon declaration of acceleration or otherwise, unless, in either case, such indebtedness is discharged, or such acceleration is rescinded, stayed or annulled, within a period of 30 calendar days after written notice of such failure is given to us by the trustee or to us and the trustee by the holders of at least 25% in aggregate principal amount of the Notes then outstanding; and



•

certain events of bankruptcy, insolvency or reorganization involving us occur and remain undischarged or unstayed for a period of 90 days.

        An Event of Default for a particular series of debt securities does not necessarily constitute an Event of Default for any other series of debt securities issued under the same or any other indenture. Within 90 days after the occurrence of any default under the indenture with respect to the Notes, the trustee shall transmit notice to the holders of such default known to the trustee, unless such default shall have been cured or waived; provided, however, that, except in the case of a default in the payment of the principal of (or premium, if any, on) or interest, if any, on any Note, the trustee shall be protected in withholding such notice if and so long as the board of directors, the executive committee or a trust committee of directors of the trustee in good faith determines that withholding of such notice is in the interest of the holders of the Notes; and provided further that in the case of any

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default or breach specified in the third bullet point above with respect to the Notes, no such notice shall be given until at least 60 days after the occurrence thereof.

        If an Event of Default occurs and is continuing, then and in every such case (other than an Event of Default specified in the last bullet point above), the trustee or the holders of not less than 25% in principal amount of the outstanding Notes may declare the entire principal amount of, and accrued and unpaid interest on, all the Notes of that series to be due and immediately payable, by a notice in writing to us (and to the trustee if given by the holders), and upon any such declaration such principal amount and accrued and unpaid interest shall become immediately due and payable. This is called a declaration of acceleration of maturity. Notwithstanding the foregoing, in the case of the events of bankruptcy, insolvency or reorganization described in the last bullet point above, 100% of the principal amount of, and accrued and unpaid interest on, the Notes will automatically become due and payable.

        At any time after a declaration of acceleration with respect to the Notes has been made and before a judgment or decree for payment of the money due has been obtained by the trustee, the holders of a majority in principal amount of the outstanding Notes, by written notice to us and the trustee, may rescind and annul such declaration and its consequences if (i) we have paid or deposited with the trustee a sum sufficient to pay all overdue installments of interest, if any, on all outstanding Notes, the principal of (and premium, if any, on) all outstanding Notes that have become due otherwise than by such declaration of acceleration and interest thereon at the rate or rates borne by or provided for in such Notes, to the extent that payment of such interest is lawful interest upon overdue installments of interest at the rate or rates borne by or provided for in such Notes, and all sums paid or advanced by the trustee and the reasonable compensation, expenses, disbursements and advances of the trustee, its agents and counsel, and (ii) all Events of Default with respect to the Notes, other than the nonpayment of the principal of (or premium, if any, on) or interest on such Notes that have become due solely by such declaration of acceleration, have been cured or waived. No such rescission will affect any subsequent default or impair any right consequent thereon.

        The trustee is not required to exercise any of the rights or powers vested in it by the indenture at the request or direction of any of the holders of the Notes unless such holders shall have offered to the trustee reasonable security or indemnity against the costs, expenses, and liabilities that might be incurred by it in compliance with such request or direction. Subject to the foregoing, the holders of a majority in principal amount of the outstanding Notes shall have the right to direct the time, method, and place of conducting any proceeding for any remedy available to the trustee or exercising any trust or power conferred on the trustee with respect to the Notes, provided that (i) such direction shall not be in conflict with any rule of law or with the indenture, (ii) the trustee may take any other action deemed proper by the trustee that is not inconsistent with such direction and (iii) the trustee need not take any action which might involve it in personal liability or be unjustly prejudicial to the holders of the Notes not consenting. No delay or omission in exercising any right or remedy will be treated as a waiver of that right, remedy, or Event of Default.

        Before you are allowed to bypass the trustee and bring your own lawsuit or other formal legal action or take other steps to enforce your rights or protect your interests relating to the debt securities, the following must occur:

•

you must give the trustee written notice that an Event of Default has occurred and remains uncured;



•

the holders of not less than 25% in principal amount of the outstanding Notes must make a written request to the trustee to take action because of the default and must offer reasonable indemnity, security, or both to the trustee against the cost, expenses, and liabilities of taking that action;

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•

the trustee must not have taken action for 60 days after receipt of the above notice and offer of indemnity and/or security; and



•

the holders of a majority in principal amount of the Notes must not have given the trustee a direction inconsistent with the above notice during that 60-day period.

        However, you are entitled at any time to bring a lawsuit for the payment of money due on your debt securities on or after the due date.

        Holders of a majority in principal amount of the Notes may waive any past defaults other than:

•

in respect of the payment of principal, any premium or interest; or



•

in respect of a covenant that cannot be modified or amended with the consent of the holder of each outstanding Note affected.

        Book-entry and other indirect holders should consult their banks or brokers for information on how to give notice or direction to or make a request of the trustee and how to declare or cancel an acceleration of maturity.

        Each year, we will furnish to each trustee a written statement of certain of our officers certifying that to their knowledge we are in compliance with the indenture governing the Notes, or else specifying any default.

Satisfaction and Discharge; Defeasance

        We may satisfy and discharge our obligations under the indenture by delivering to the security registrar for cancellation all outstanding Notes or by depositing with the trustee or delivering to the holders, as applicable, after the Notes have become due and payable, or otherwise, moneys sufficient to pay all of the outstanding Notes and paying all other sums payable under the indenture by us. Such discharge is subject to terms contained in the indenture.

        In addition, the Notes are subject to defeasance and covenant defeasance, in each case, in accordance with the terms of the indenture. "Covenant defeasance" refers to our ability, under current United States federal tax law and the indenture, to be released from some of the restrictive covenants in the indenture if certain conditions are satisfied. See "Description of Our Debt Securities—Defeasance—Covenant Defeasance" in the accompanying prospectus for more information. "Defeasance" or "full defeasance" refers to our ability, if there is a change in United States federal tax law or if we obtain an IRS ruling, to legally release ourselves from all payment and other obligations on the Notes if we put in place certain arrangements for you to be repaid. See "Description of Our Debt Securities—Defeasance—Full Defeasance" in the accompanying prospectus for more information.

Trustee

        The Bank of New York Mellon Trust Company, N.A., is the trustee, security registrar and paying agent. The Bank of New York Mellon Trust Company, N.A., in each of its capacities, including without limitation as trustee, security registrar and paying agent, assumes no responsibility for the accuracy or completeness of the information concerning us or our affiliates or any other party contained in this document or the related documents or for any failure by us or any other party to disclose events that may have occurred and may affect the significance or accuracy of such information, or for any information provided to it by us, including but not limited to settlement amounts and any other information.

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Governing Law

        The indenture provides that it and the Notes shall be governed by, and construed in accordance with, the laws of the State of New York without regard to principles of conflicts of laws.

Book-Entry, Settlement and Clearance

Global Notes

        The Notes will be initially issued in the form of one or more registered Notes in global form, without interest coupons, or the Global Notes. Upon issuance, each of the Global Notes will be deposited with the trustee as custodian for DTC and registered in the name of Cede & Co., as nominee of DTC.

        Ownership of beneficial interests in a Global Note will be limited to persons who have accounts with DTC, or the DTC participants, or persons who hold interests through DTC participants. We expect that under procedures established by DTC:

•

upon deposit of a Global Note with DTC's custodian, DTC will credit portions of the principal amount of the Global Note to the accounts of the DTC participants designated by the underwriters; and



•

ownership of beneficial interests in a Global Note will be shown on, and transfer of ownership of those interests will be effected only through, records maintained by DTC (with respect to interests of DTC participants) and the records of DTC participants (with respect to other owners of beneficial interests in the Global Note).

        Beneficial interests in Global Notes may not be exchanged for Notes in physical, certificated form except in the limited circumstances described below.

Book-Entry Procedures for Global Notes

        All interests in the Global Notes will be subject to the operations and procedures of DTC. We provide the following summary of those operations and procedures solely for the convenience of investors. The operations and procedures of DTC are controlled by that settlement system and may be changed at any time. Neither we nor the underwriters are responsible for those operations or procedures.

        DTC has advised us that it is:

•

a limited purpose trust company organized under the laws of the State of New York;



•

a "banking organization" within the meaning of the New York State Banking Law;



•

a member of the Federal Reserve System;



•

a "clearing corporation" within the meaning of the Uniform Commercial Code; and



•

a "clearing agency" registered under Section 17A of the Exchange Act.

        DTC was created to hold securities for its participants and to facilitate the clearance and settlement of securities transactions between its participants through electronic book-entry changes to the accounts of its participants. DTC's participants include securities brokers and dealers, including the underwriters; banks and trust companies; clearing corporations and other organizations. Indirect access to DTC's system is also available to others such as banks, brokers, dealers and trust companies; these indirect participants clear through or maintain a custodial relationship with a DTC participant, either directly or indirectly. Investors who are not DTC participants may beneficially own securities held by or on behalf of DTC only through DTC participants or indirect participants in DTC.

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        So long as DTC's nominee is the registered owner of a Global Note, that nominee will be considered the sole owner or holder of the Notes represented by that Global Note for all purposes under the indenture. Except as provided below, owners of beneficial interests in a Global Note:

•

will not be entitled to have Notes represented by the Global Note registered in their names;



•

will not receive or be entitled to receive physical, certificated Notes; and



•

will not be considered the owners or holders of the Notes under the indenture for any purpose, including with respect to the giving of any direction, instruction or approval to the trustee under the indenture.

        As a result, each investor who owns a beneficial interest in a Global Note must rely on the procedures of DTC to exercise any rights of a holder of Notes under the indenture (and, if the investor is not a participant or an indirect participant in DTC, on the procedures of the DTC participant through which the investor owns its interest).

        Payments of principal and interest with respect to the Notes represented by a Global Note will be made by the trustee to DTC's nominee as the registered holder of the Global Note. Neither we nor the trustee will have any responsibility or liability for the payment of amounts to owners of beneficial interests in a Global Note, for any aspect of the records relating to or payments made on account of those interests by DTC, or for maintaining, supervising or reviewing any records of DTC relating to those interests.

        Payments by participants and indirect participants in DTC to the owners of beneficial interests in a Global Note will be governed by standing instructions and customary industry practice and will be the responsibility of those participants or indirect participants and DTC.

        Transfers between participants in DTC will be effected under DTC's procedures and will be settled in same-day funds.

Certificated Notes

        Notes in physical, certificated form will be issued and delivered to each person that DTC identifies as a beneficial owner of the related Notes only if:

•

DTC notifies us at any time that it is unwilling or unable to continue as depositary for the Global Notes and a successor depositary is not appointed within 90 days; or



•

DTC ceases to be registered as a clearing agency under the Exchange Act and a successor depositary is not appointed within 90 days.

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CERTAIN U.S. FEDERAL INCOME TAX CONSEQUENCES

        The following discussion is a summary of certain material U.S. federal income tax consequences relevant to the purchase, ownership and disposition of the Notes, but does not purport to be a complete analysis of all potential tax consequences. The discussion is based upon the Internal Revenue Code of 1986, as amended (the "Code"), the regulations promulgated thereunder by the U.S. Treasury (the "Treasury Regulations"), rulings and pronouncements issued by the Internal Revenue Service (the "IRS"), and judicial decisions, all as of the date hereof and all of which are subject to change at any time. Any such change may be applied retroactively in a manner that could adversely affect a holder of the Notes. We have not sought any ruling from the IRS with respect to the statements made and the conclusions reached in the following discussion, and there can be no assurance that the IRS will agree with such statements and conclusions.

        This discussion does not address all of the U.S. federal income tax consequences that may be relevant to a holder in light of such holder's particular circumstances or to holders subject to special rules, including, without limitation:

•

banks, insurance companies and other financial institutions;



•

individual retirement accounts and other tax-deferred accounts;



•

regulated investment companies and real estate investment trusts;



•

U.S. expatriates and certain former citizens or long-term residents of the United States;



•

holders subject to the alternative minimum tax;



•

dealers in securities or currencies;



•

traders in securities;



•

partnerships, S corporations or other pass-through entities;



•

U.S. holders (as defined below) whose functional currency is not the U.S. dollar;



•

controlled foreign corporations;



•

tax-exempt organizations;



•

passive foreign investment companies;



•

persons holding the Notes as part of a "straddle," "hedge," "conversion transaction" or other risk reduction transaction; and



•

persons deemed to sell the Notes under the constructive sale provisions of the Code.

        In addition, this discussion is limited to persons purchasing the Notes for cash at original issue and at their original "issue price" within the meaning of Section 1273 of the Code (i.e., the first price at which a substantial amount of the Notes are sold to the public for cash). Moreover, the effects of other U.S. federal tax laws (such as estate and gift tax laws) and any applicable state, local or foreign tax laws are not discussed. The discussion deals only with Notes held as "capital assets" within the meaning of Section 1221 of the Code. This discussion also does not address the U.S. federal income tax consequences to beneficial owners of the Notes that are subject to the special tax accounting rules under Section 451(b) of the Code.

        If an entity taxable as a partnership holds the Notes, the tax treatment of an owner of the entity generally will depend on the status of the particular owner in question and the activities of the entity. Owners of any such entity should consult their tax advisors as to the specific tax consequences to them of holding the Notes indirectly through ownership of such entity.

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        YOU ARE URGED TO CONSULT YOUR TAX ADVISOR WITH RESPECT TO THE APPLICATION OF THE U.S. FEDERAL INCOME TAX LAWS TO YOUR PARTICULAR SITUATION AS WELL AS ANY TAX CONSEQUENCES OF THE PURCHASE, OWNERSHIP AND DISPOSITION OF THE NOTES ARISING UNDER THE U.S. FEDERAL ESTATE OR GIFT TAX LAWS OR UNDER THE LAWS OF ANY STATE, LOCAL, FOREIGN OR OTHER TAXING JURISDICTION OR UNDER ANY APPLICABLE INCOME TAX TREATY.

U.S. Holders

        The following is a summary of the material U.S. federal income tax consequences that will apply to you if you are a "U.S. holder" of a Note. As used herein, "U.S. holder" means a beneficial owner of a Note who is for U.S. federal income tax purposes:

•

an individual who is a citizen or resident of the United States, including an alien individual who is a lawful permanent resident of the United States or meets the "substantial presence" test under Section 7701(b) of the Code;



•

a corporation or other entity taxable as a corporation created or organized in or under the laws of the United States, any state thereof, or the District of Columbia;



•

an estate, the income of which is subject to U.S. federal income tax regardless of its source; or



•

a trust, if a U.S. court can exercise primary supervision over the administration of the trust and one or more "United States persons" within the meaning of Section 7701(a)(30) of the Code can control all substantial trust decisions, or, if the trust was in existence on August 20, 1996, and it has elected to continue to be treated as a United States person.

Payments of Interest

        The following discussion assumes that the Notes will be issued with no original issue discount or a de minimis amount of original issue discount for U.S. federal income tax purposes. Stated interest on the Notes generally will be taxable to a U.S. holder as ordinary income at the time that such interest is received or accrued, in accordance with such U.S. holder's method of tax accounting for U.S. federal income tax purposes.

Sale or Other Taxable Disposition of Notes

        A U.S. holder will recognize gain or loss on the sale, exchange, redemption, retirement or other taxable disposition of a Note equal to the difference between the amount realized upon the disposition (less any portion allocable to any accrued and unpaid interest, which will be taxable as interest to the extent not previously included in income) and the U.S. holder's adjusted tax basis in the Note. A U.S. holder's adjusted tax basis in a Note generally will be equal to the amount that the U.S. holder paid for the Note less any principal payments received by the U.S. holder. Any gain or loss will be a capital gain or loss, and will be a long-term capital gain or loss if the U.S. holder has held the Note for more than one year at the time of disposition. Otherwise, such gain or loss will be a short-term capital gain or loss. Long-term capital gains recognized by certain non-corporate U.S. holders, including individuals, are currently subject to a reduced tax rate. The deductibility of capital losses is subject to limitations.

Information Reporting and Backup Withholding

        A U.S. holder may be subject to information reporting and backup withholding when such U.S. holder receives interest payments on the Notes held or upon the proceeds received upon the sale or other disposition of such Notes (including a redemption or retirement of the Notes). Certain U.S.

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holders generally are not subject to information reporting or backup withholding. A U.S. holder will be subject to backup withholding if such U.S. holder is not otherwise exempt and such U.S. holder:

•

fails to furnish the U.S. holder's taxpayer identification number ("TIN"), which, for an individual, ordinarily is his or her social security number;



•

furnishes an incorrect TIN;



•

is notified by the IRS that the U.S. holder has failed properly to report payments of interest or dividends; or



•

fails to certify, under penalties of perjury, on an IRS Form W-9 (Request for Taxpayer Identification Number and Certification) or a suitable substitute form (or other applicable certificate), that the U.S. holder has furnished a correct TIN and that the IRS has not notified the U.S. holder that the U.S. holder is subject to backup withholding.

        U.S. holders should consult their tax advisors regarding their qualification for an exemption from backup withholding and the procedures for obtaining such an exemption, if applicable. Backup withholding is not an additional tax, and taxpayers may use amounts withheld as a credit against their U.S. federal income tax liability or may claim a refund if they timely provide certain information to the IRS.

Unearned Income Medicare Contribution

        A tax of 3.8% will be imposed on certain "net investment income" (or "undistributed net investment income", in the case of estates and trusts) received by individuals with adjusted modified gross incomes in excess of $200,000 ($250,000 in the case of married individuals filing jointly) and certain estates and trusts. "Net investment income" as defined for U.S. federal Medicare contribution purposes generally includes interest payments and gain recognized from the sale or other disposition of the Notes. Tax-exempt trusts, which are not subject to income taxes generally, and foreign individuals will not be subject to this tax. U.S. holders should consult their own tax advisors regarding the effect, if any, of this tax on their ownership and disposition of the Notes.

Non-U.S. Holders

        The following is a summary of certain material U.S. federal income tax consequences that will apply to you if you are a "Non-U.S. holder" of a Note. A "Non-U.S. holder" is a beneficial owner of a Note who is not a U.S. holder or a partnership for U.S. federal income tax purposes. Special rules may apply to Non-U.S. holders that are subject to special treatment under the Code, including controlled foreign corporations, passive foreign investment companies, U.S. expatriates, and foreign persons eligible for benefits under an applicable income tax treaty with the U.S. Such Non-U.S. holders should consult their tax advisors to determine the U.S. federal, state, local and other tax consequences that may be relevant to them including any reporting requirements.

Payments of Interest

        Generally, interest income paid to a Non-U.S. holder that is not effectively connected with the Non-U.S. holder's conduct of a U.S. trade or business is subject to withholding tax at a rate of 30% (or, if applicable, a lower treaty rate). Nevertheless, subject to the discussion below concerning FATCA, interest paid on a Note to a Non-U.S. holder that is not effectively connected with the Non-U.S. holder's conduct of a U.S. trade or business generally will not be subject to U.S. federal withholding tax provided that:

•

such Non-U.S. holder does not directly or indirectly own 10% or more of the total combined voting power of all classes of our voting stock;

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•

such Non-U.S. holder is not a controlled foreign corporation that is related to us through actual or constructive stock ownership and is not a bank that received such Note on an extension of credit made pursuant to a loan agreement entered into in the ordinary course of its trade or business; and



•

either (1) the Non-U.S. holder certifies in a statement provided to us or the paying agent generally on IRS Form W-8BEN, or IRS Form W-8BEN-E, under penalties of perjury, that it is the beneficial owner of the Notes and not a "United States person" within the meaning of the Code and provides its name and address, (2) a securities clearing organization, bank or other financial institution that holds customers' securities in the ordinary course of its trade or business and holds the Note on behalf of the Non-U.S. holder certifies to us or the paying agent under penalties of perjury that it, or the financial institution between it and the Non-U.S. holder, has received from the Non-U.S. holder a statement generally on IRS Form W-8BEN, or IRS Form W-8BEN-E, under penalties of perjury, that such Non-U.S. holder is the beneficial owner of the Notes and is not a United States person and provides us or the paying agent with a copy of such statement or (3) the Non-U.S. holder holds its Note directly through a "qualified intermediary" and certain conditions are satisfied.

        Even if the above conditions are not met, a Non-U.S. holder generally will be entitled to a reduction in or an exemption from withholding tax on interest if the Non-U.S. holder provides us or our paying agent with a properly executed IRS Form W-8BEN or IRS Form W-8BEN-E, as applicable, or a suitable substitute form (or other applicable certificate) claiming an exemption from or reduction of the withholding tax under the benefit of an income tax treaty between the United States and the Non-U.S. holder's country of residence. A Non-U.S. holder is required to inform the recipient of any change in the information on such statement within 30 days of such change. Special certification rules apply to Non-U.S. holders that are pass-through entities rather than corporations or individuals.

        If interest paid to a Non-U.S. holder is effectively connected with the Non-U.S. holder's conduct of a U.S. trade or business, then, the Non-U.S. holder will be exempt from U.S. federal withholding tax, so long as the Non-U.S. holder has provided an IRS Form W-8ECI or substantially similar substitute form stating that the interest that the Non-U.S. holder receives on the Notes is effectively connected with the Non-U.S. holder's conduct of a trade or business in the United States. In such a case, a Non-U.S. holder will be subject to tax on the interest it receives on a net income basis in the same manner as if such Non-U.S. holder were a U.S. holder. In addition, if the Non-U.S. holder is a foreign corporation, such interest may be subject to a branch profits tax at a rate of 30% or lower applicable treaty rate.

Sale or Other Taxable Disposition of Notes

        Subject to the discussion below regarding FATCA, any gain realized by a Non-U.S. holder on the sale, exchange, retirement, redemption or other taxable disposition of a Note generally will not be subject to U.S. federal income tax unless:

•

the gain is effectively connected with the Non-U.S. holder's conduct of a trade or business in the United States (and, if required by an applicable income tax treaty, the Non-U.S. holder maintains a U.S. permanent establishment to which such gain is attributable); or



•

the Non-U.S. holder is an individual who is present in the United States for 183 days or more in the taxable year of sale, exchange or other disposition, certain conditions are met and the Non-U.S. holder is not eligible for relief under an applicable income tax treaty.

        A Non-U.S. holder described in the first bullet point above will be required to pay U.S. federal income tax on the net gain derived from the sale or other taxable disposition generally in the same manner as if such Non-U.S. holder were a U.S. holder, and if such Non-U.S. holder is a foreign

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corporation, it may also be required to pay an additional branch profits tax at a 30% rate (or a lower rate if so specified by an applicable income tax treaty). A Non-U.S. holder described in the second bullet point above will be subject to U.S. federal income tax at a rate of 30% (or, if applicable, a lower treaty rate) on the gain derived from the sale or other taxable disposition, which may be offset by certain U.S. source capital losses, even though the Non-U.S. holder is not considered a resident of the United States.

        Certain other exceptions may be applicable, and Non-U.S. holders should consult their own tax advisors with regard to whether taxes will be imposed on capital gain in their individual circumstances.

Information Reporting and Backup Withholding

        The amount of interest that we pay to any Non-U.S. holder on the Notes will be reported to the Non-U.S. holder and to the IRS annually on an IRS Form 1042-S, regardless of whether any tax was actually withheld. Copies of these information returns may also be made available under the provisions of a specific income tax treaty or agreement to the tax authorities of the country in which the Non-U.S. holder resides. However, a Non-U.S. holder generally will not be subject to backup withholding and certain other information reporting with respect to payments that we make to the Non-U.S. holder, provided that we do not have actual knowledge or reason to know that such Non-U.S. holder is a "United States person," within the meaning of the Code, and the Non-U.S. holder has given us the statement described above under "Non-U.S. holders—Payments of Interest."

        If a Non-U.S. holder sells or exchanges a Note through a United States broker or the United States office of a foreign broker, the proceeds from such sale or exchange will be subject to information reporting and backup withholding unless the Non-U.S. holder provides a withholding certificate or other appropriate documentary evidence establishing that such holder is not a U.S. holder to the broker and such broker does not have actual knowledge or reason to know that such holder is a U.S. holder, or the Non-U.S. holder is an exempt recipient eligible for an exemption from information reporting and backup withholding. If a Non-U.S. holder sells or exchanges a Note through the foreign office of a broker who is a United States person or has certain enumerated connections with the United States, the proceeds from such sale or exchange will be subject to information reporting unless the Non-U.S. holder provides to such broker a withholding certificate or other documentary evidence establishing that such holder is not a U.S. holder and such broker does not have actual knowledge or reason to know that such evidence is false, or the Non-U.S. holder is an exempt recipient eligible for an exemption from information reporting. In circumstances where information reporting by the foreign office of such a broker is required, backup withholding will be required only if the broker has actual knowledge that the holder is a U.S. holder.

        A Non-U.S. holder generally will be entitled to credit any amounts withheld under the backup withholding rules against the Non-U.S. holder's U.S. federal income tax liability or may claim a refund provided that the required information is furnished to the IRS in a timely manner.

        Non-U.S. holders are urged to consult their tax advisors regarding the application of information reporting and backup withholding in their particular situations, the availability of an exemption therefrom, and the procedures for obtaining such an exemption, if available.

Foreign Account Tax Compliance Act

        The Foreign Account Tax Compliance Act, or FATCA, and U.S. Treasury Regulations and other IRS administrative guidance thereunder generally impose a U.S. federal withholding tax of 30% on U.S. source interest on a debt obligation and the gross proceeds from the disposition of a debt obligation of a type that produces U.S. source interest, which, in each case, would include the Notes, to certain non-U.S. entities (including, in some circumstances, where such an entity is acting as an intermediary) that fail to comply with certain certification and information reporting requirements,

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including reporting requirements regarding its United States account holders (in the case of foreign financial institutions) or beneficial United States owners (in the case of non-financial foreign entities). However, under current proposed regulations promulgated by the Treasury Department, withholding under FATCA does not apply to gross proceeds from any sale or disposition of a Note. Taxpayers may generally rely on those proposed regulations until final regulations are issued. Foreign financial institutions located in jurisdictions that have an intergovernmental agreement with the United States with respect to FATCA may be subject to different rules. In addition, under certain circumstances, a Non-U.S. holder might be eligible for refunds or credits of any taxes imposed pursuant to FATCA. Prospective investors in the Notes should consult their own tax advisors regarding the effect, if any, of the FATCA rules for them based on their particular circumstances.

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UNDERWRITING (CONFLICTS OF INTEREST)

        Under the terms and subject to the conditions contained in an underwriting agreement dated January     , 2021, the underwriters named below, for whom RBC Capital Markets, LLC is acting as representative, have severally agreed to purchase, and we have agreed to sell to them, the aggregate principal amount of Notes indicated below:

        The underwriting agreement provides that the obligations of the underwriters to pay for and accept delivery of the Notes offered hereby are subject to the approval of certain legal matters by their counsel and to certain other conditions. The underwriters are severally obligated to take and pay for all Notes offered hereby if any such Notes are taken. The underwriters reserve the right to withdraw, cancel or modify offers to the public and to reject orders in whole or in part. We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the Securities Act.

Commissions and Discounts

        An underwriting discount of        % per Note will be paid by us.

        The following table shows the total underwriting discounts and commissions that we are to pay to the underwriters in connection with this offering.

        The underwriters propose to offer some of the Notes to the public at the public offering price set forth on the cover page of this prospectus supplement and some of the Notes to certain other broker-dealers at the public offering price less a concession not in excess of        % of the aggregate principal amount of the Notes. The underwriters may allow, and the dealers may reallow, a discount not in excess of        % of the aggregate principal amount of the Notes. After the initial offering of the Notes to the public, the public offering price and such concessions may be changed. No such change shall change the amount of proceeds to be received by us as set forth on the cover page of this prospectus supplement. The expenses of the offering, not including the underwriting discount, are estimated at $300,000 and are payable by us.

Alternative Settlement Cycle

        We expect that delivery of the Notes will be made against payment therefore on or about January     , 2021, which will be the third business day following the trade date for the issuance of the Notes (such settlement being herein referred to as "T+3"). Under Rule 15c6-1 promulgated under the Exchange Act, trades in the secondary market generally are required to settle in two business days, unless the parties to any such trade expressly agree otherwise. Accordingly, purchasers who wish to trade the Notes prior to the date of delivery hereunder will be required, by virtue of the fact that the Notes initially will settle in T+3 business days, to specify an alternative settlement arrangement at the time of any such trade to prevent a failed settlement.

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No Sales of Similar Securities

        Subject to certain exceptions, we have agreed not to directly or indirectly, offer, pledge, sell, contract to sell, grant any option for the sale of, or otherwise transfer or dispose of any debt securities issued or guaranteed by the Company that are substantially similar to the Notes or any securities convertible into or exercisable or exchangeable for such debt securities for a period of 30 days after the date of this prospectus supplement without first obtaining the written consent of the representative. This consent may be given at any time without public notice.

Listing

        The Notes are a new issue of securities with no established trading market. The Notes will not be listed on any securities exchange or quoted on any automated dealer quotation system.

        We have been advised by the underwriters that they presently intend to make a market in the Notes after completion of the offering as permitted by applicable laws and regulations. The underwriters are not obligated, however, to make a market in the Notes and any such market-making may be discontinued at any time in the sole discretion of the underwriters without any notice. Accordingly, no assurance can be given as to the liquidity of, or development of a public trading market for, the Notes. If an active public trading market for the Notes does not develop, the market price and liquidity of the Notes may be adversely affected.

Price Stabilization and Short Positions

        In connection with the offering, the underwriters may purchase and sell Notes in the open market. These transactions may include short sales, purchases to cover positions created by short sales and stabilizing transactions. Short sales involve the sale by the underwriters of a greater number of Notes than required to be purchased in this offering. Covering transactions involve purchases of the Notes in the open market after the distribution has been completed in order to cover short positions. Stabilizing transactions consist of certain bids or purchases of Notes made for the purpose of preventing or retarding a decline in the market price of the Notes while the offering is in progress.

        The underwriters also may impose a penalty bid. This occurs when a particular underwriter repays to the underwriters a portion of the underwriting discount received by it because the representative has repurchased Notes sold by or for the account of such underwriter in stabilizing or short covering transactions.

        Any of these activities may cause the price of the Notes to be higher than the price that otherwise would exist in the open market in the absence of such transactions. These transactions may be discontinued at any time without any notice relating thereto.

Conflicts of Interest

        Affiliates of RBC Capital Markets, LLC, Truist Securities, Inc., SMBC Nikko Securities America, Inc. and Raymond James & Associates, Inc., underwriters in this offering, act as lenders and/or agents under our Credit Facility. Certain of the net proceeds from the sale of the Notes, not including underwriting compensation, may be paid to such affiliates of RBC Capital Markets, LLC, Truist Securities, Inc., SMBC Nikko Securities American, Inc. and Raymond James & Associates, Inc. in connection with the repayment of debt owed under our Credit Facility. As a result, RBC Capital Markets, LLC, Truist Securities, Inc., SMBC Nikko Securities America, Inc. and Raymond James & Associates, Inc. and/or their affiliates may receive more than 5% of the net proceeds of this offering, not including underwriting compensation.

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        The underwriters and/or their affiliates from time to time provide and may in the future provide investment banking, commercial banking and financial advisory services to us, for which they have received and may receive customary compensation.

        In addition, the underwriters and/or their affiliates may from time to time refer investment banking clients to us as potential portfolio investments. If we invest in those clients, we may indirectly utilize net proceeds from this offering to fund such investments, and the referring underwriter or its affiliate may receive placement fees from its client in connection with such financing, which placement fees may be paid out of the amount funded by us.

        The addresses of the underwriters are: RBC Capital Markets, LLC, 200 Vesey Street, 8th Floor, New York, New York 10281; Truist Securities, Inc., 3333 Peachtree Road NE, Atlanta, Georgia 30326; SMBC Nikko Securities America, Inc., 277 Park Avenue, New York, New York 10172; and Raymond James & Associates, Inc., 880 Carillon Parkway, St. Petersburg, Florida 33716.

Selling Restrictions

European Economic Area

        The Notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the European Economic Area ("EEA"). For these purposes, a retail investor means a person who is one (or more) of: (i) a retail client as defined in point (11) of Article 4(1) of Directive 2014/65/EU (as amended, "MiFID II"); or (ii) a customer within the meaning of Directive 2002/92/EC (as amended, the "Insurance Mediation Directive"), where that customer would not qualify as a professional client as defined in point (10) of Article 4(1) of MiFID II; or (iii) not a qualified investor as defined in Directive 2003/71/EC (as amended, the "Prospectus Directive"). Consequently, no key information document required by Regulation (EU) No 1286/2014 (as amended, the "PRIIPs Regulation") for offering or selling the Notes or otherwise making them available to retail investors in the EEA has been prepared and therefore offering or selling the Notes or otherwise making them available to any retail investor in the EEA may be unlawful under the PRIIPS Regulation. This prospectus supplement and accompanying prospectus has been prepared on the basis that any offer of the Notes in any Member State of the EEA will be made pursuant to an exemption under the Prospectus Directive from the requirement to publish a prospectus for offers of the Notes. This prospectus supplement and accompanying prospectus is not a prospectus for the purposes of the Prospectus Directive.

United Kingdom

        Each underwriter has represented and agreed that:

(a)

it has only communicated or caused to be communicated and will only communicate or cause to be communicated an invitation or inducement to engage in investment activity (within the meaning of Section 21 of the Financial Services and Markets Act 2000 (FSMA)) received by it in connection with the issue or sale of the Notes in circumstances in which Section 21(1) of the FSMA does not apply to the Issuer; and



(b)

it has complied and will comply with all applicable provisions of the FSMA with respect to anything done by it in relation to the Notes in, from or otherwise involving the United Kingdom.

Switzerland

        This prospectus supplement may only be freely circulated, and securities of the issuer may only be freely offered or sold, to regulated financial intermediaries such as banks, securities dealers, fund management companies, asset managers of collective investment schemes and central banks as well as

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to regulated insurance companies. Circulating this prospectus supplement and offering or selling the Notes to other persons or entities including qualified investors as defined in the Federal Act on Collective Investment Schemes ("CISA") and its implementing Ordinance ("CISO") may trigger, in particular, (i) licensing/prudential supervision requirements for the distributor and/or the issuer, (ii) a requirement to appoint a representative and paying agent in Switzerland and (iii) the necessity of a written distribution agreement between the representative in Switzerland and the distributor. Accordingly, legal advice should be sought before providing this prospectus supplement to and offering or selling the Notes to any other persons or entities. This prospectus supplement does not constitute an issuance prospectus pursuant to Articles 652a or 1156 of the Swiss Code of Obligations and may not comply with the information standards required thereunder. The Notes will not be listed on the SIX Swiss Exchange, and consequently the information presented in this document does not necessarily comply with the information standards set out in the relevant listing rules. The documentation of the issuer has not been and will not be approved by the Swiss Financial Market Supervisory Authority FINMA under the CISA. Therefore, purchasers do not benefit from protection under the CISA or supervision by the FINMA. This prospectus supplement does not constitute investment advice. It may only be used by persons to whom it has been handed out in connection with the Notes and may neither be copied or directly/indirectly distributed or made available to other persons.

Hong Kong

        The Notes may not be offered or sold by means of any document other than (i) in circumstances which do not constitute an offer to the public within the meaning of the Companies Ordinance (Cap.32, Laws of Hong Kong), or (ii) to "professional investors" within the meaning of the Securities and Futures Ordinance (Cap.571, Laws of Hong Kong) and any rules made thereunder, or (iii) in other circumstances which do not result in the document being a "prospectus" within the meaning of the Companies Ordinance (Cap.32, Laws of Hong Kong), and no advertisement, invitation or document relating to the Notes may be issued or may be in the possession of any person for the purpose of issue (in each case whether in Hong Kong or elsewhere), which is directed at, or the contents of which are likely to be accessed or read by, the public in Hong Kong (except if permitted to do so under the laws of Hong Kong) other than with respect to Notes which are or are intended to be disposed of only to persons outside Hong Kong or only to "professional investors" within the meaning of the Securities and Futures Ordinance (Cap. 571, Laws of Hong Kong) and any rules made thereunder.

Singapore

        This prospectus has not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, this prospectus and any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the Notes may not be circulated or distributed, nor may the Notes be offered or sold, or be made the subject of an invitation for subscription or purchase, whether directly or indirectly, to persons in Singapore other than (i) to an institutional investor under Section 274 of the Securities and Futures Act, Chapter 289 of Singapore (the "SFA"), (ii) to a relevant person, or any person pursuant to Section 275(1A), and in accordance with the conditions, specified in Section 275 of the SFA or (iii) otherwise pursuant to, and in accordance with the conditions of, any other applicable provision of the SFA.

        Where the Notes are subscribed or purchased under Section 275 by a relevant person which is: (a) a corporation (which is not an accredited investor) the sole business of which is to hold investments and the entire share capital of which is owned by one or more individuals, each of whom is an accredited investor; or (b) a trust (where the trustee is not an accredited investor) whose sole purpose is to hold investments and each beneficiary is an accredited investor, Notes, debentures and units of Notes and debentures of that corporation or the beneficiaries' rights and interest in that trust shall not be transferable for 6 months after that corporation or that trust has acquired the Notes under

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Section 275 except: (1) to an institutional investor under Section 274 of the SFA or to a relevant person, or any person pursuant to Section 275(1A), and in accordance with the conditions, specified in Section 275 of the SFA; (2) where no consideration is given for the transfer; or (3) by operation of law.

Japan

        The securities have not been and will not be registered under the Financial Instruments and Exchange Law of Japan (the Financial Instruments and Exchange Law) and each underwriter has agreed that it will not offer or sell any securities, directly or indirectly, in Japan or to, or for the benefit of, any resident of Japan (which term as used herein means any person resident in Japan, including any corporation or other entity organized under the laws of Japan), or to others for re-offering or resale, directly or indirectly, in Japan or to a resident of Japan, except pursuant to an exemption from the registration requirements of, and otherwise in compliance with, the Financial Instruments and Exchange Law and any other applicable laws, regulations and ministerial guidelines of Japan.

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LEGAL MATTERS

        Certain legal matters regarding the Notes offered hereby will be passed upon for us by Dechert LLP, Washington D.C., and certain legal matters in connection with this offering will be passed upon for the underwriters by Fried, Frank, Harris, Shriver & Jacobson LLP, New York, New York.

AVAILABLE INFORMATION

        This prospectus supplement and the accompanying prospectus constitute part of a universal shelf registration statement on Form N-2 that we have filed with the SEC, together with any and all amendments and related exhibits, under the Securities Act. The registration statement contains additional information about us, our securities and the Notes being offered by this prospectus supplement. Statements contained herein concerning any document filed as an exhibit are not necessarily complete, and, in each instance, reference is made to the copy of such document filed as an exhibit to the registration statement. Each such statement is qualified in its entirety by such reference.

        We file with or submit to the SEC annual, quarterly and current reports, proxy statements and other information meeting the informational requirements of the Exchange Act. The SEC maintains an Internet site that contains reports, proxy and information statements and other information filed electronically by us with the SEC, which are available free of charge on the SEC's website at www.sec.gov. This information is also available free of charge by contacting us at 1300 Post Oak Boulevard, 8th Floor, Houston, Texas 77056 or by telephone at (713) 350-6000 or on our website at www.mainstcapital.com. Information contained on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus, and you should not consider that information to be part of this prospectus supplement or the accompanying prospectus.

INCORPORATION BY REFERENCE

        We incorporate by reference in this prospectus supplement the documents listed below and any reports and other documents we file with the SEC pursuant to Sections 13(a), 13(c), 14, or 15(d) of the Exchange Act after the date of this prospectus supplement and prior to the termination of this offering (such reports and other documents deemed to be incorporated by reference into this prospectus supplement and to be part hereof from the date of filing of such reports and other documents); provided, however, that any document, report, exhibit (or portion of any of the foregoing) or other information "furnished" to the SEC pursuant to the Exchange Act shall not be incorporated by reference into this prospectus supplement:

•

our Annual Report on Form 10-K for the fiscal year ended December 31, 2019 (filed with the SEC on February 28, 2020);



•

our Quarterly Reports on Form 10-Q for the fiscal quarters ended March 31, 2020 (filed with the SEC on May 8, 2020), June 30, 2020 (filed with the SEC on August 7, 2020), and September 30, 2020 (filed with the SEC on November 6, 2020);



•

our Definitive Proxy Statement on Schedule 14A, filed with the SEC on March 23, 2020; and



•

our Current Reports on Form 8-K (other than information furnished rather than filed) filed with the SEC on January 7, 2020, February 26, 2020, March 4, 2020, March 24, 2020, April 15, 2020, May 5, 2020, May 6, 2020, July 20, 2020, July 24, 2020, August 4, 2020, November 3, 2020, and November 4, 2020.

        Any statement contained in any document incorporated or deemed to be incorporated by reference herein shall be deemed to be modified or superseded for purposes of this prospectus supplement and the accompanying prospectus to the extent that a statement contained in this prospectus supplement or in any other subsequently filed document which also is or is deemed to be

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incorporated by reference in this prospectus supplement modifies or supersedes such earlier statement. Any statement so modified or superseded shall not be deemed, except as so modified or superseded, to constitute a part of this prospectus supplement.

        We will provide without charge to each person, including any beneficial owner, to whom a copy of this prospectus supplement is delivered, upon written or oral request of any such person, a copy of any or all of the information that has been incorporated by reference in this prospectus supplement but not delivered with this prospectus supplement, excluding exhibits to a document unless an exhibit has been specifically incorporated by reference in that document. To obtain copies of these filings, see "Available Information" in this prospectus supplement.

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PROSPECTUS

$1,500,000,000

Common Stock
Preferred Stock
Subscription Rights
Debt Securities

           We may offer, from time to time in one or more offerings, up to $1,500,000,000 of our common stock, preferred stock, subscription rights or debt securities, which we refer to, collectively, as the "securities." Our securities may be offered at prices and on terms to be disclosed in one or more supplements to this prospectus. The offering price per share of our common stock, less any underwriting commissions or discounts, will not be less than the net asset value per share of our common stock at the time of the offering, except (i) with the requisite approval of our common stockholders or (ii) under such other circumstances as the Securities and Exchange Commission may permit. We did not seek stockholder authorization to issue common stock at a price below net asset value per share at our 2018 annual meeting of stockholders, and we are not seeking such approval at our 2019 annual meeting of stockholders, because our common stock price per share has been trading significantly above the current net asset value per share of our common stock, but we may seek such authorization at future annual meetings or special meetings of stockholders. Sales of common stock at prices below net asset value per share dilute the interests of existing stockholders, have the effect of reducing our net asset value per share and may reduce our market price per share. In addition, we have received stockholder approval to issue warrants, options or rights to subscribe for, convert to, or purchase shares of our common stock at a price per share below the net asset value per share subject to the applicable requirements of the Investment Company Act of 1940, as amended. There is no expiration date on our ability to issue such warrants, options, rights or convertible securities based on this stockholder approval. Moreover, continuous sales of common stock below net asset value may have a negative impact on total returns and could have a negative impact on the market price of our shares of common stock. See "Sales of Common Stock Below Net Asset Value."

           Shares of closed-end investment companies such as us frequently trade at a discount to their net asset value. This risk is separate and distinct from the risk that our net asset value per share may decline. We cannot predict whether our common stock will trade above, at or below net asset value. You should read carefully the information contained or incorporated by reference in this prospectus, the applicable prospectus supplement and in any free writing prospectuses we have authorized for use in connection with a specific offering before you invest in our common stock.

           Our securities may be offered to one or more purchasers directly by us, through agents designated from time to time by us, or to or through underwriters or dealers. The prospectus supplement relating to the offering will identify any agents or underwriters involved in the sale of our securities, and will disclose any applicable purchase price, fee, commission or discount arrangement between us and our agents or underwriters or among our underwriters or the basis upon which such amount may be calculated. See "Plan of Distribution."

           We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market ("LMM") companies and debt capital to middle market ("Middle Market") companies. Our LMM companies generally have annual revenues between $10 million and $150 million, and our LMM portfolio investments generally range in size from $5 million to $50 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies, with annual revenues typically between $150 million and $1.5 billion, and our Middle Market investments generally range in size from $3 million to $20 million.

           The LMM and Middle Market securities in which we invest generally would be rated below investment grade if they were rated by rating agencies. Below investment grade securities, which are often referred to as "junk," have predominantly speculative characteristics with respect to the issuer's capacity to pay interest and repay principal. They may also be difficult to value and are illiquid.

           Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company.

           We are an internally managed, closed-end, non-diversified management investment company that has elected to be treated as a business development company under the Investment Company Act of 1940, as amended.

           Our common stock is listed on the New York Stock Exchange ("NYSE") under the symbol "MAIN." On April 29, 2019, the last reported sale price of our common stock on the NYSE was $39.27 per share, and the net asset value per share of our common stock on December 31, 2018 (the last date prior to the date of this prospectus on which we determined our net asset value per share) was $24.09.

           Investing in our securities involves a high degree of risk, and should be considered highly speculative. You should review carefully the risks and uncertainties, including the risk of leverage and dilution, described in the section titled "Risk Factors" included in, or incorporated by reference into, the applicable prospectus supplement and in any free writing prospectuses we have authorized for use in connection with a specific offering, and under similar headings in the other documents that are incorporated by reference into this prospectus before investing in our securities.

           This prospectus describes some of the general terms that may apply to an offering of our securities. We will provide the specific terms of these offerings and securities in one or more supplements to this prospectus. We may also authorize one or more free writing prospectuses to be provided to you in connection with these offerings. The prospectus supplement and any related free writing prospectus may also add, update, or change information contained in this prospectus. You should carefully read this prospectus, the applicable prospectus supplement, and any related free writing prospectus, and the documents incorporated by reference, before buying any of the securities being offered. We file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission, or SEC. This information is available free of charge by contacting us at 1300 Post Oak Boulevard, 8^th Floor, Houston, Texas 77056 or by telephone at (713) 350-6000 or on our website at www.mainstcapital.com. Information contained on our website is not incorporated by reference into this prospectus, and you should not consider that information to be part of this prospectus. The SEC also maintains a website at www.sec.gov that contains such information.

           Neither the Securities and Exchange Commission nor any state securities commission, nor any other regulatory body, has approved or disapproved of these securities or determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

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        This prospectus is part of an automatic registration statement that we have filed with the Securities and Exchange Commission, or SEC, using the "shelf" registration process as a "well-known seasoned issuer" as defined in Rule 405 under the Securities Act of 1933, as amended (the "Securities Act"). Under this shelf registration statement, we may offer, from time to time in one or more offerings, up to $1,500,000,000 of our securities, either individually or in combination with other securities described in this prospectus, on terms to be determined at the time of the offering. This prospectus provides you with a general description of the securities that we may offer. Each time we use this prospectus to offer securities, we will provide a prospectus supplement that will contain specific information about the terms of that offering. We may also authorize one or more free writing prospectuses to be provided to you that may contain material information relating to these offerings. In a prospectus supplement or free writing prospectus, we may also add, update, or change any of the information contained in this prospectus or in the documents we have incorporated by reference into this prospectus. This prospectus, together with the applicable prospectus supplement, any related free writing prospectus, and the documents incorporated by reference into this prospectus and the applicable prospectus supplement, will include all material information relating to the applicable offering. Before buying any of the securities being offered, you should carefully read both this prospectus and the applicable prospectus supplement and any related free writing prospectus, together with the additional information described in the section titled "Available Information."

        This prospectus may contain estimates and information concerning our industry that are based on industry publications and reports. This information involves many assumptions and limitations, and you are cautioned not to give undue weight to these estimates. We have not independently verified the accuracy or completeness of the data contained in these industry publications and reports. The industry in which we operate is subject to a high degree of uncertainty and risk due to a variety of factors, including those described in the section titled "Risk Factors," that could cause results to differ materially from those expressed in these publications and reports.

        This prospectus includes summaries of certain provisions contained in some of the documents described in this prospectus, but reference is made to the actual documents for complete information. All of the summaries are qualified in their entirety by the actual documents. Copies of some of the

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documents referred to herein have been filed, will be filed, or will be incorporated by reference as exhibits to the registration statement of which this prospectus is a part, and you may obtain copies of those documents as described in the section titled "Available Information."

        You should rely only on the information included or incorporated by reference in this prospectus, any prospectus supplement or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We have not authorized any dealer, salesperson or other person to provide you with different information or to make representations as to matters not stated in this prospectus, any prospectus supplement or in any free writing prospectus prepared by or on behalf of us or to which we have referred you. We take no responsibility for, and can provide no assurance as to the reliability of, any other information that others may give you. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus, any applicable prospectus supplement and any free writing prospectus prepared by or on behalf of us or to which we have referred you do not constitute an offer to sell, or a solicitation of an offer to buy, any securities by any person in any jurisdiction where it is unlawful for that person to make such an offer or solicitation or to any person in any jurisdiction to whom it is unlawful to make such an offer or solicitation. You should not assume that the information included or incorporated by reference in this prospectus or any prospectus supplement or in any such free writing prospectus is accurate as of any date other than their respective dates.

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RISK FACTORS

        Investing in our securities involves a high degree of risk. Before deciding whether to invest in our securities, you should carefully consider the risks and uncertainties described in the section titled "Risk Factors" in the applicable prospectus supplement and any related free writing prospectus, and discussed in the section titled "Risk Factors" in our most recent Annual Report on Form 10-K, as well as in subsequent filings with the SEC, which are incorporated by reference into this prospectus in their entirety, together with other information in this prospectus, the documents incorporated by reference, and any free writing prospectus that we may authorize for use in connection with this offering. The risks described in these documents are not the only ones we face. Additional risks and uncertainties that we are unaware of, or that we currently believe are not material, may also become important factors that adversely affect our business. Past financial performance may not be a reliable indicator of future performance, and historical trends should not be used to anticipate results or trends in future periods. If any of these risks actually occurs, our business, reputation, financial condition, results of operations, revenue, and future prospects could be seriously harmed. This could cause our net asset value and the trading price of our securities to decline, resulting in a loss of all or part of your investment. Please also read carefully the section titled "Cautionary Statement Concerning Forward-Looking Statements."

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CAUTIONARY STATEMENT CONCERNING FORWARD-LOOKING STATEMENTS

        This prospectus, including the documents that we incorporate by reference herein, contains, and any applicable prospectus supplement or free writing prospectus, including the documents we incorporate by reference therein, may contain forward-looking statements, including statements regarding our future financial condition, business strategy, and plans and objectives of management for future operations. All statements other than statements of historical facts, including statements regarding our future results of operations or financial condition, business strategy and plans, and objectives of management for future operations, are forward-looking statements. The forward-looking statements contained or incorporated by reference in this prospectus and any applicable prospectus supplement or free writing prospectus may include statements as to:

•

our future operating results and dividend projections;



•

our business prospects and the prospects of our portfolio companies;



•

the impact of the investments that we expect to make;



•

the ability of our portfolio companies to achieve their objectives;



•

our expected financings and investments;



•

the adequacy of our cash resources and working capital; and



•

the timing of cash flows, if any, from the operations of our portfolio companies.

        In some cases, you can identify forward-looking statements because they contain words such as "anticipate," "believe," "contemplate," "continue," "could," "estimate," "expect," "intend," "may," "plan," "potential," "predict," "project," "should," "target," "will," or "would" or the negative of these words or other similar terms or expressions, although not all forward-looking statements include these words or expressions. The forward-looking statements contained or incorporated by reference in this prospectus and any applicable prospectus supplement or free writing prospectus involve risks and uncertainties. Our actual results could differ materially from those implied or expressed in the forward-looking statements for any reason, including the factors set forth in "Risk Factors" in our most recent Annual Report on Form 10-K, as well as in subsequent filings with the SEC, and elsewhere contained or incorporated by reference in this prospectus and any applicable prospectus supplement or free writing prospectus. Other factors that could cause actual results to differ materially include:

•

changes in the economy;



•

risks associated with possible disruption in our operations or the economy generally due to terrorism or natural disasters; and



•

future changes in laws or regulations and conditions in our operating areas.

        Discussions containing these forward-looking statements may be found in the sections titled "Business," "Risk Factors," and "Management's Discussion and Analysis of Financial Condition and Results of Operations" incorporated by reference from our most recent Annual Report on Form 10-K, as well as in subsequent filings with the SEC. We discuss in greater detail, and incorporate by reference into this prospectus in their entirety, many of these risks and uncertainties in the sections titled "Risk Factors" in the applicable prospectus supplement, in any free writing prospectus we may authorize for use in connection with a specific offering, and in our most recent Annual Report on Form 10-K, as well as in subsequent filings with the SEC. In addition, statements that we "believe" and similar statements reflect our beliefs and opinions on the relevant subject. These statements are based on information available to us as of the applicable date of this prospectus, free writing prospectus and documents incorporated by reference into this prospectus, and while we believe such information forms a reasonable basis for such statements, such information may be limited or incomplete, and our statements should not be read to indicate that we have conducted an exhaustive inquiry into, or review of, all potentially available relevant information. These statements are inherently uncertain and investors are cautioned not to unduly rely on these statements.

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USE OF PROCEEDS

        Except as described in any applicable prospectus supplement or in any free writing prospectuses we have authorized for use in connection with a specific offering, we intend to use the net proceeds from any offering pursuant to this prospectus to make investments in accordance with our investment objective and strategies, to pay our operating expenses and other cash obligations, and for general corporate purposes. Our ability to achieve our investment objective may be limited to the extent that the net proceeds from an offering, pending full investment, are held in interest-bearing deposits or other short-term instruments. See "Risk Factors—Risks Relating to Our Securities—We may be unable to invest a significant portion of the net proceeds from an offering or from exiting an investment or other capital on acceptable terms, which could harm our financial condition and operating results" in our most recently filed Annual Report on Form 10-K, as well as in subsequent filings with the SEC.

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PRICE RANGE OF COMMON STOCK

        Our common stock is traded on the New York Stock Exchange ("NYSE") under the symbol "MAIN."

        The following table sets forth, for the periods indicated, the range of high and low closing prices of our common stock as reported on the NYSE, and the sales price as a percentage of the net asset value per share of our common stock.

        On April 29, 2019, the last sale price of our common stock on the NYSE was $39.27 per share, and there were approximately 365 holders of record of the common stock which did not include stockholders for whom shares are held in "nominee" or "street name." The net asset value per share of our common stock on December 31, 2018 (the last date prior to the date of this prospectus on which we determined our net asset value per share) was $24.09, and the premium of the April 29, 2019 closing price of our common stock was 63% to this net asset value per share.

        Shares of BDCs may trade at a market price that is less than the value of the net assets attributable to those shares. The possibility that our shares of common stock will trade at a discount from net asset value per share or at premiums that are unsustainable over the long term are separate and distinct from the risk that our net asset value per share will decrease. It is not possible to predict whether our common stock will trade at, above, or below net asset value per share. Since our IPO in October 2007, our shares of common stock have traded at prices both less than and exceeding our net asset value per share.

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SENIOR SECURITIES

        Information about our senior securities is shown in the following table as of December 31 for the years indicated in the table, unless otherwise noted. Grant Thornton LLP's report on the senior securities table as of December 31, 2018, is an exhibit to the registration statement of which this prospectus is a part.

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PORTFOLIO COMPANIES

        The following table sets forth certain unaudited information as of December 31, 2018 (dollars in thousands), for the portfolio companies in which we had a debt or equity investment. Other than these investments, our only formal relationships with our portfolio companies are the managerial assistance ancillary to our investments and the board observer or participation rights we may receive. As of December 31, 2018, none of our portfolio company investments constituted five percent or more of our total assets. The following table excludes our investments in marketable securities and idle funds investments.