Filed Pursuant to Rule 424(b)(2)

 Registration Statement No. 333-263258 

PROSPECTUS SUPPLEMENT

(to Prospectus dated March 3, 2022)

1,170,000 Shares

Common Stock

We are offering for sale 1,170,000 shares of our common stock.

We are an internally managed, closed end, non-diversified management investment company that has elected to be treated as a business development company (“BDC”) under the Investment Company Act of 1940, as amended. Our principal investment objective is to maximize our portfolio’s total return by generating current income from our debt investments and current income 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.

The 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 common stock is listed on the New York Stock Exchange under the symbol “MAIN.” On August 10, 2022, the last reported sale price of our common stock on the New York Stock Exchange was $ 44.89 per share, and the net asset value per share of our common stock on June 30, 2022 (the last date prior to the date of this prospectus supplement on which we determined our net asset value per share) was $25.37.

Investing in our common stock involves a high degree of risk and should be considered highly speculative. You should carefully review the risks and uncertainties, including the risk of leverage and dilution, described in the section titled “Risk Factors” beginning on page S-10 of this prospectus supplement, page 9 of the accompanying prospectus, in our most recently filed Annual Report on Form 10-K and 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 before investing in our common stock.

This prospectus supplement and the accompanying prospectus contain important information about us that a prospective investor should know before investing in our common stock. 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 the documents incorporated by reference, before investing in our common stock. 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, 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.

The underwriters have the option to purchase up to an additional 175,500 shares of common stock at the public offering price, less the underwriting discount, within 30 days from the date of this prospectus supplement. If the option to purchase additional shares is exercised in full, the total public offering price will be $57,654,675, the total underwriting discount (4.00%) will be $2,306,187, and the total proceeds to us, before deducting estimated expenses payable by us of $200,000, will be $ 55,348,488.

Neither the SEC 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 underwriters expect to deliver the shares on or about August 15, 2022.

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The date of this prospectus supplement is August 11, 2022

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TABLE OF CONTENTS

Prospectus Supplement

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Prospectus

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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 common stock 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, the accompanying prospectus or in any free writing prospectus prepared by or on behalf of us that relates to this offering of common stock. 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 common stock. 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 common stock do not constitute an offer to sell, or a solicitation of an offer to buy, any shares of our common stock 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 of any sale of our securities.

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 common stock 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 section titled “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 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 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 our common stock.

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PROSPECTUS SUMMARY

This summary highlights information included elsewhere or incorporated by reference in this prospectus supplement or the accompanying prospectus. It is not complete and may not contain all of the information that you should consider before making your investment decision. To understand the terms of the common stock offered hereby before making your investment decision, you should carefully read this entire prospectus supplement, the accompanying prospectus, any free writing prospectus relating to this offering and the documents incorporated by reference in this prospectus supplement and the accompanying prospectus, as provided in “Available Information” and “Incorporation by Reference” beginning on page S-12 of this prospectus supplement and on page 95 of the accompanying prospectus.

Organization

Main Street Capital Corporation (“MSCC”) is 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. The portfolio investments of MSCC and its consolidated subsidiaries are typically made to support management buyouts, recapitalizations, growth financings, refinancings and acquisitions of companies that operate in a variety of industry sectors. MSCC seeks to partner with entrepreneurs, business owners and management teams and generally provides “one stop” financing alternatives within its LMM investment strategy. MSCC and its consolidated subsidiaries invest primarily in secured debt investments, equity investments, warrants and other securities of LMM companies based in the United States and in secured debt investments of Middle Market companies generally headquartered in the United States.

MSCC was formed in March 2007 to operate as an internally managed business development company (“BDC”) under the Investment Company Act of 1940, as amended (the “1940 Act”). Because MSCC is internally managed, all of the executive officers and other employees are employed by MSCC. Therefore, MSCC does not pay any external investment advisory fees, but instead directly incurs the operating costs associated with employing investment and portfolio management professionals.

MSCC wholly owns several investment funds, including Main Street Mezzanine Fund, LP (“MSMF”) and Main Street Capital III, LP (“MSC III” and, together with MSMF, the “Funds”), and each of their general partners. The Funds are each licensed as a Small Business Investment Company (“SBIC”) by the United States Small Business Administration (“SBA”).

MSC Adviser I, LLC (the “External Investment Manager”) was formed in November 2013 as a wholly owned subsidiary of MSCC to provide investment management and other services to parties other than MSCC and its subsidiaries or their portfolio companies (“External Parties”) and receives fee income for such services. MSCC has been granted no-action relief by the Securities and Exchange Commission (“SEC”) to allow the External Investment Manager to register as a registered investment adviser under the Investment Advisers Act of 1940, as amended. Since the External Investment Manager conducts all of its investment management activities for External Parties, it is accounted for as a portfolio investment of MSCC and is not included as a consolidated subsidiary of MSCC in MSCC’s consolidated financial statements.

MSCC has elected to be treated for U.S. federal income tax purposes as a regulated investment company (“RIC”) under Subchapter M of the Internal Revenue Code of 1986, as amended (the “Code”). As a result, MSCC generally does not pay corporate-level U.S. federal income taxes on any net ordinary taxable income or capital gains that it distributes to its stockholders.

MSCC has certain direct and indirect wholly owned subsidiaries that have elected to be taxable entities (the “Taxable Subsidiaries”). The primary purpose of the Taxable Subsidiaries is to permit MSCC to hold equity investments in portfolio companies which are “pass-through” entities for tax purposes.

Unless otherwise noted or the context otherwise indicates, the terms “we,” “us,” “our,” the “Company” and “Main Street” refer to MSCC and its consolidated subsidiaries, which include the Funds and the Taxable Subsidiaries.

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Overview

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You should be aware that our LMM portfolio investments, Private Loan portfolio investments and Middle Market portfolio investments carry a number of risks including: (1) investing in companies which may have limited operating histories and financial resources; (2) holding investments that generally are not publicly traded and which may be subject to legal and other restrictions on resale; and (3) other risks common to investing in below investment grade debt and equity investments in our investment portfolio. Please see “Risk Factors — Risks Related to our Investments” in our most recently filed Annual Report on Form 10-K, as well as in subsequent filings with the SEC, for a more complete discussion of the risks involved with investing in our investment portfolio.

Our principal executive offices are located at 1300 Post Oak Boulevard, 8th Floor, Houston, Texas 77056. We maintain a website on the Internet 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.

Risks Associated with Our Business

Our business is subject to numerous risks, as described in the section titled “Risk Factors” in the accompanying prospectus and in any free writing prospectuses we have authorized for use in connection with this offering, and under similar headings in the documents that are incorporated by reference into this prospectus supplement and the accompanying prospectus, including the section titled “Risk Factors” included in our most recent Annual Report on Form 10-K, as well as in subsequent filings with the SEC.

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Recent Developments

In August 2022, we declared a supplemental cash dividend of $0.10 per share payable in September 2022. This supplemental cash dividend is in addition to the previously announced regular monthly cash dividends that we declared for the third quarter of 2022 of $0.215 per share for each of July, August and September 2022, or total monthly cash dividends of $0.645 per share for the quarter.

In August 2022, we declared regular monthly dividends of $0.22 per share for each of October, November and December of 2022. These regular monthly dividends equal a total of $0.66 per share for the fourth quarter of 2022, representing a 4.8% increase from the regular monthly dividends paid in the fourth quarter of 2021. Including the regular monthly and supplemental dividends declared for the third and fourth quarters of 2022, we will have paid $35.02 per share in cumulative dividends since our October 2007 initial public offering.

During August 2022, we amended our multi-year revolving credit facility (the “Credit Facility”) to, among other changes, (i) increase the revolving commitments by lenders to $920.0 million, and with the right to request an increase in commitments under the Credit Facility from new and existing lenders on the same terms and conditions as the existing commitments up to a total of $1.4 billion, subject to certain conditions, (ii) extend the revolving period under the Credit Facility to August 4, 2026 and the final maturity date of the Credit Facility to August 4, 2027 and (iii) transition the reference rate used to determine the borrowing interest rate from LIBOR to Term SOFR (“SOFR”) plus an applicable credit spread adjustment, while maintaining the interest rate spread of 1.875%.

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The Offering

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FEES AND EXPENSES

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Example

The following example demonstrates the projected dollar amount of total cumulative expenses that would be incurred over various periods with respect to a hypothetical investment in our common stock. In calculating the following expense amounts, we have assumed we would have no additional leverage and that our annual operating

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expenses would remain at the levels set forth in the table above, and that you would pay a sales load of up to 4.00% (the commission to be paid by us with respect to common stock sold by us in this offering).

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While the example assumes, as required by the SEC, a 5.0% annual return, our performance will vary and may result in a return greater or less than 5.0%. In addition, while the example assumes reinvestment of all dividends at net asset value, participants in our dividend reinvestment plan will receive a number of shares of our common stock, determined by dividing the total dollar amount of the dividend payable to a participant by (i) the market price per share of our common stock at the close of trading on a valuation date determined by our Board of Directors for each dividend in the event that we use newly issued shares to satisfy the share requirements of the dividend reinvestment plan or (ii) the average purchase price of all shares of common stock purchased by the plan administrator in the event that shares are purchased in the open market to satisfy the share requirements of the dividend reinvestment plan, which may be at, above or below net asset value. See “Dividend Reinvestment and Direct Stock Purchase Plan” in the accompanying prospectus for additional information regarding our dividend reinvestment plan.

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This example and the expenses in the table above should not be considered a representation of our future expenses, and actual expenses (including the cost of debt, if any, and other expenses) may be greater or less than those shown.

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USE OF PROCEEDS

The net proceeds from the sale of the 1,170,000 shares of common stock in this offering are approximately $47.9 million, and approximately $55.1 million if the underwriters’ option to purchase additional shares is exercised in full, 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 (i) to make investments in accordance with our investment objective and strategies described in this prospectus supplement, the accompanying prospectus and any free writing prospectus relating to this offering, (ii) 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, (iii) to pay our operating expenses and other cash obligations, and (iv) for general corporate purposes.

On August 8, 2022, we had approximately $580.0 million outstanding under our Credit Facility. Our Credit Facility matures in August 2027, unless extended, and bears interest, at our election, on a per annum basis at a rate equal to the applicable SOFR rate plus an applicable credit spread adjustment 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, an underwriter 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 this Credit Facility, and such affiliates therefore may receive a portion of the proceeds from this offering through the repayment of those borrowings. See “Plan of Distribution” below.

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CAPITALIZATION

The following table sets forth our capitalization:

This table should be read in conjunction with “Use of Proceeds” included in this prospectus supplement and “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 June 30, 2022, incorporated by reference herein.

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PRICE RANGE OF COMMON STOCK

Our common stock is traded on the 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.

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On August 8, 2022, the last sale price of our common stock on the NYSE was $44.50 per share, and there were approximately 371 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 June 30, 2022 (the last date prior to the date of this prospectus on which we determined our net asset value per share) was $25.37, and the premium of the August 8, 2022 closing price of our common stock was 75% 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 initial public offering 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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UNDERWRITING

Under the terms and subject to the conditions contained in an underwriting agreement dated August 11, 2022, the underwriters named below, for whom RBC Capital Markets is acting as representative, have severally agreed to purchase, and we have agreed to sell to them, the number of shares of common stock indicated below:

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The underwriting agreement provides that the obligations of the underwriters to pay for and accept delivery of the shares of common stock 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 shares of common stock offered hereby (other than those covered by the underwriters’ option to purchase additional shares described below) if any such shares are taken. The offering of the common stock by the underwriters is subject to receipt and acceptance and subject to the underwriters’ right to reject any order in whole or in part. We have agreed to indemnify the several underwriters against certain liabilities, including liabilities under the Securities Act.

Our common stock is listed on the New York Stock Exchange under the symbol “MAIN.”

Option to Purchase Additional Shares

We have granted to the underwriters an option, exercisable for 30 days from the date of this prospectus supplement, to purchase up to an aggregate of 175,500 additional shares of common stock at the price set forth on the cover page of this prospectus supplement. To the extent such option is exercised, each underwriter will become obligated, subject to certain conditions, to purchase approximately the same percentage of such additional shares of common stock as the number set forth next to such underwriter’s name in the preceding table bears to the total number of shares set forth next to the names of all underwriters in the preceding table.

Lock-Up Agreements

We, and certain of our executive officers and directors, have agreed with the underwriters, subject to certain exceptions, not to issue, offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase, lend or otherwise transfer or dispose of, directly or indirectly, any shares of our common stock or other capital stock, or any securities convertible into or exercisable or exchangeable for any shares of our common stock or other capital stock or any right to acquire shares of our common stock or other capital stock, during the period from the date of this prospectus supplement continuing through and including the date that is 30 days after the date of this prospectus supplement. The representative, at any time and without notice, may release all or any portion of the common stock subject to the foregoing lock-up agreements.

Underwriting Discounts

The underwriters initially propose to offer the shares directly to the public at the public offering price set forth on the cover page of this prospectus supplement and to certain dealers at a price that represents a concession not in excess of $1.0284 per share below the public offering price. After the initial public offering of the shares, the offering price and other selling terms may be changed by the underwriters.

The following table provides information regarding the per share and total underwriting discount that we are to pay to the underwriters. These amounts are shown assuming both no exercise and full exercise of the underwriters’ option to purchase up to 175,500 additional shares from us.

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We will pay all expenses incident to the offering and sale of shares of our common stock by us in this offering. We estimate that the total expenses of the offering, excluding the underwriting discount will be approximately $200,000.

A prospectus supplement in electronic format may be made available on the web sites maintained by one or more of the underwriters, or selling group members, if any, participating in this offering. The representative may agree to allocate a number of shares to underwriters and selling group members for the sale to their online brokerage account holders. Internet distributions will be allocated by the underwriters and selling group members that will make Internet distributions on the same basis as other allocations. The representative may agree to allocate a number of shares to underwriters for sale to their online brokerage account holders.

Price Stabilization, Short Positions and Penalty Bids

In connection with this offering, the underwriters may purchase and sell shares of our common stock in the open market. These transactions may include over- allotment, syndicate covering transactions and stabilizing transactions. An over-allotment involves syndicate sales of shares in excess of the number of shares to be purchased by the underwriters in the offering, which creates a syndicate short position. Syndicate covering transactions involve purchases of shares in the open market after the distribution has been completed in order to cover syndicate short positions.

Stabilizing transactions consist of some bids or purchases of shares of our common stock made for the purpose of preventing or slowing a decline in the market price of the shares while the offering is in progress.

In addition, the underwriters may impose penalty bids, under which they may reclaim the selling concession from a syndicate member when the shares of our common stock originally sold by that syndicate member are purchased in a stabilizing transaction or syndicate covering transaction to cover syndicate short positions.

Similar to other purchase transactions, these activities may have the effect of raising or maintaining the market price of the common stock or preventing or slowing a decline in the market price of the common stock. As a result, the price of the common stock may be higher than the price that might otherwise exist in the open market. Except for the sale of shares of our common stock in this offering, the underwriters may carry out these transactions on the New York Stock Exchange, in the over-the-counter market or otherwise.

Neither the underwriters nor we make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of the shares. In addition, neither the underwriters nor we make any representation that the underwriters will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

Electronic Offer, Sale and Distribution of Shares

The underwriters may make prospectuses available in electronic (PDF) format. A prospectus in electronic (PDF) format may be made available on a web site maintained by the underwriters, and the underwriters may distribute such prospectuses electronically. The underwriters may allocate a limited number of shares for sale to their online brokerage customers.

Conflicts of Interest

Affiliates of RBC Capital Markets, LLC, an underwriter in this offering, act as a lenders under our Credit Facility. Certain of the net proceeds from the sale of the shares of common stock, not including underwriting compensation, may be paid to such affiliates of RBC Capital Markets, LLC in connection with the repayment of

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debt owed under our Credit Facility. As a result, RBC Capital Markets, LLC and/or its affiliates may receive more than 5% of the net proceeds of this offering, not including underwriting compensation.

The underwriters and/or their affiliates from time to time provide and may in the future provide investment banking, commercial banking, corporate trust, custodial, treasury, 401(k) and financial advisory services to us, for which they have received and may receive customary compensation. The underwriters and/or their affiliates from time to time provide and may in the future provide similar services to our portfolio companies.

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; UBS Securities LLC, 1285 Avenue of the Americas, New York, New York 10019; and Wells Fargo Securities, LLC, 550 South Tryon Street, Charlotte, NC 28202.

Notice to Prospective Investors in Canada

No prospectus has been filed with any securities commission or similar regulatory authority in Canada in connection with the offer and sale of the securities offering herein and no securities commission or similar regulatory authority in Canada has reviewed or in any way passed upon this document or on the merits of the securities and any representation to the contrary is an offence. The offer and sale of the common stock in Canada is being made on a private placement basis only and is exempt from the requirement that the issuer prepares and files a prospectus under applicable Canadian securities laws. Any resale of common stock acquired by a Canadian investor in this offering must be made in accordance with applicable Canadian securities laws, which resale restrictions may under certain circumstances apply to resales of the common stock outside of Canada.

As applicable, each Canadian investor who purchases the common stock will be deemed to have represented to the issuer, the underwriters and to each dealer from whom a purchase confirmation is received, as applicable, that the investor (i) is purchasing as principal, or is deemed to be purchasing as principal in accordance with applicable Canadian securities laws, for investment only and not with a view to resale or redistribution; (ii) is an “accredited investor” as such term is defined in section 1.1 of National Instrument 45-106 Prospectus Exemptions (“NI 45-106”) or, in Ontario, as such term is defined in section 73.3(1) of the Securities Act (Ontario); and (iii) is a “permitted client” as such term is defined in section 1.1 of National Instrument 31-103 Registration Requirements, Exemptions and Ongoing Registrant Obligations.

Securities legislation in certain provinces or territories of Canada may provide a purchaser with remedies for rescission or damages if this prospectus supplement (including any amendment thereto) contains a misrepresentation, provided that the remedies for rescission or damages are exercised by the purchaser within the time limit prescribed by securities legislation of the purchaser’s province or territory. The purchaser should refer to any applicable provisions of the securities legislation of the purchaser’s province or territory for particulars of these rights or consult with a legal advisor.

Pursuant to section 3A.3 of National Instrument 33-105 Underwriting Conflicts (“NI 33-105”), this offering is conducted pursuant to any exemption from the requirement that Canadian investors be provided with certain underwriter conflicts of interest disclosure that would otherwise be required pursuant to subsection 2.1(1) of NI 33-105.

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LEGAL MATTERS

Certain legal matters regarding the shares of common stock 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.

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

The consolidated financial statements, financial highlights and Schedule 12-14 of Main Street Capital Corporation appearing in our Annual Report on Form 10-K for the year ended December 31, 2021 have been audited by Grant Thornton LLP, an independent registered public accounting firm, and incorporated in this prospectus supplement by reference. Such consolidated financial statements are incorporated by reference in reliance on the report of Grant Thornton LLP given on their authority as experts in accounting and auditing. The senior securities table of Main Street Capital Corporation, incorporated by reference into the accompanying prospectus and elsewhere in the registration statement, has been so included in reliance upon the report of Grant Thornton LLP, an independent registered public accounting firm.

Grant Thornton LLP’s principal business address is Grant Thornton Tower, 171 North Clark, Suite 200, Chicago, Illinois, 60601.

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 and our shares of common stock 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 Securities Exchange Act of 1934, as amended (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.

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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:

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 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

Common Stock

Preferred Stock

Subscription Rights

Debt Securities

Main Street Capital Corporation is a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market (“LMM”) companies and debt capital through our private loan and middle market investment strategies. Our LMM investment strategy involves investments in companies that generally have annual revenues between $10 million and $150 million and our LMM portfolio investments generally range in size from $5 million to $75 million. Our Middle Market investment strategy involves investments in companies that are generally larger in size than our LMM 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 $25 million. Our Private Loan investment strategy involves investments in companies that are consistent with the size of the companies in our LMM and Middle Market investment strategies, and our Private Loan investments generally range in size from $10 million to $75 million.

We are an internally managed, closed-end, non-diversified management investment company that has elected to be treated as a business development company (“BDC”) under the Investment Company Act of 1940, as amended. 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 may offer, from time to time in one or more offerings or series, 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. In the event we offer common stock, 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) in connection with a rights offering to our existing stockholders, (ii) with the prior approval of the majority of our common stockholders or (iii) under such circumstances as the U.S. Securities and Exchange Commission (the “SEC”) may permit. 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.

Investing in our securities involves a high degree of risk and should be considered highly speculative. You should carefully review 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 this prospectus, the applicable prospectus supplement and in any free writing prospectuses we have authorized for use in connection with a specific offering.

The LMM, Private Loan 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 common stock is listed on the New York Stock Exchange (“NYSE”) under the symbol “MAIN.” On March 1, 2022, the last reported sale price of our common stock on the NYSE was $42.75 per share, and the net asset

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value per share of our common stock on December 31, 2021 (the last date prior to the date of this prospectus on which we determined our net asset value per share) was $25.29.

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 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 SEC 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.

This prospectus may not be used to consummate sales of securities unless accompanied by a prospectus supplement.

The date of this prospectus is March 3, 2022

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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 or series, our securities 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

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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 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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PROSPECTUS SUMMARY

This summary highlights information included elsewhere in this prospectus or incorporated by reference. It is not complete and may not contain all of the information that you should consider before making your investment decision. You should carefully read the entire prospectus, the applicable prospectus supplement, and any related free writing prospectus, including the risks of investing in our securities discussed in the section titled “Risk Factors” in the applicable prospectus supplement and any related free writing prospectus, and under similar headings in the other documents that are incorporated by reference into this prospectus. Before making your investment decision, you should also carefully read the information incorporated by reference into this prospectus, including our financial statements and related notes, and the exhibits to the registration statement of which this prospectus is a part. Any yield information contained or incorporated by reference in this prospectus related to debt investments in our investment portfolio is not intended to approximate a return on your investment in us and does not take into account other aspects of our business, including our operating and other expenses, or other costs incurred by you in connection with your investment in us.

Organization

Main Street Capital Corporation (“MSCC”) is 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. The portfolio investments of MSCC and its consolidated subsidiaries are typically made to support management buyouts, recapitalizations, growth financings, refinancings and acquisitions of companies that operate in a variety of industry sectors. MSCC seeks to partner with entrepreneurs, business owners and management teams and generally provides “one stop” financing alternatives within its LMM portfolio. MSCC and its consolidated subsidiaries invest primarily in secured debt investments, equity investments, warrants and other securities of LMM companies based in the United States and in secured debt investments of Middle Market companies generally headquartered in the United States.

MSCC was formed in March 2007 to operate as an internally managed business development company (“BDC”) under the Investment Company Act of 1940, as amended (the “1940 Act”). MSCC wholly owns several investment funds, including Main Street Mezzanine Fund, LP (“MSMF”) and Main Street Capital III, LP (“MSC III” and, together with MSMF, the “Funds”), and each of their general partners. The Funds are each licensed as a Small Business Investment Company (“SBIC”) by the United States Small Business Administration (“SBA”). Because MSCC is internally managed, all of the executive officers and other employees are employed by MSCC. Therefore, MSCC does not pay any external investment advisory fees, but instead directly incurs the operating costs associated with employing investment and portfolio management professionals.

MSC Adviser I, LLC (the “External Investment Manager”) was formed in November 2013 as a wholly owned subsidiary of MSCC to provide investment management and other services to parties other than MSCC and its subsidiaries or their portfolio companies (“External Parties”) and receives fee income for such services. MSCC has been granted no-action relief by the SEC to allow the External Investment Manager to register as a registered investment adviser under the Investment Advisers Act of 1940, as amended (the “Advisers Act”). Since the External Investment Manager conducts all of its investment management activities for External Parties, it is accounted for as a portfolio investment of MSCC and is not included as a consolidated subsidiary of MSCC in MSCC’s consolidated financial statements.

MSCC has elected to be treated for U.S. federal income tax purposes as a regulated investment company (“RIC”) under Subchapter M of the Internal Revenue Code of 1986, as amended (the “Code”). As a result, MSCC generally will not pay corporate-level U.S. federal income taxes on any net ordinary taxable income or capital gains that it distributes to its stockholders.

MSCC has certain direct and indirect wholly owned subsidiaries that have elected to be taxable entities (the “Taxable Subsidiaries”). The primary purpose of the Taxable Subsidiaries is to permit MSCC to hold equity investments in portfolio companies which are “pass-through” entities for tax purposes.

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Unless otherwise noted or the context otherwise indicates, the terms “we,” “us,” “our,” the “Company” and “Main Street” refer to MSCC and its consolidated subsidiaries, which include the Funds and the Taxable Subsidiaries.

Overview

Our principal investment objective is to maximize our portfolio’s total return by generating current income from our debt investments and current income 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 seek to achieve our investment objective through our LMM, Private Loan and Middle Market investment strategies.

Our LMM investment strategy involves investments in companies that generally have annual revenues between $10 million and $150 million and our LMM portfolio investments generally range in size from $5 million to $75 million. Our Middle Market investment strategy involves investments in companies that are generally larger in size than our LMM 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 $25 million. Our private loan (“Private Loan”) investment strategy involves investments in companies that are consistent with the size of the companies in our LMM and Middle Market investment strategies, and our Private Loan investments generally range in size from $10 million to $75 million.

We seek to fill the financing gap for LMM businesses, which, historically, have had limited access to financing from commercial banks and other traditional sources. The underserved nature of the LMM creates the opportunity for us to meet the financing needs of LMM companies while also negotiating favorable transaction terms and equity participations. Our ability to invest across a company’s capital structure, from secured loans to equity securities, allows us to offer portfolio companies a comprehensive suite of financing options, or a “one stop” financing solution. Providing customized, “one stop” financing solutions is important to LMM portfolio companies. We generally seek to partner directly with entrepreneurs, management teams and business owners in making our investments. Our LMM portfolio debt investments are generally secured by a first lien on the assets of the portfolio company and typically have a term of between five and seven years from the original investment date.

Private Loan investments consist generally of loans that have been originated directly by us or through strategic relationships with other investment funds on a collaborative basis and are often referred to in the debt markets as “club deals.” Private Loan investments are typically similar in size, structure, terms and conditions to investments we hold in our LMM portfolio and Middle Market portfolio. Our Private Loan portfolio debt investments are generally secured by a first priority lien on the assets of the portfolio company and typically have a term of between three and seven years from the original investment date.

Our Middle Market portfolio investments primarily consist of direct investments in or secondary purchases of interest-bearing syndicated loans or debt securities in privately held companies based in the United States that are generally larger in size than the companies included in our LMM portfolio. Our Middle Market portfolio debt investments are generally secured by a first priority lien on the assets of the portfolio company and typically have an expected duration of between three and seven years from the original investment date.

Our other portfolio (“Other Portfolio”) investments primarily consist of investments that are not consistent with the typical profiles for our LMM, Private Loan or Middle Market portfolio investments, including investments which may be managed by third parties. In our Other Portfolio, we may incur indirect fees and expenses in connection with investments managed by third parties, such as investments in other investment companies or private funds.

Subject to changes in our cash and overall liquidity, our investment portfolio may also include short-term portfolio investments that are atypical of our LMM, Middle Market and Private Loan portfolio investments in that they are intended to be a short-term deployment of capital. These assets are typically expected to be liquidated in one year or less and are not expected to be a significant portion of the overall investment portfolio.

Our external asset management business is conducted through the External Investment Manager. The External Investment Manager earns management fees based on the assets of the funds under management and may earn incentive fees, or a carried interest, based on the performance of the funds managed.

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Our portfolio investments are generally made through MSCC, the Taxable Subsidiaries and the Funds. MSCC, the Taxable Subsidiaries and the Funds share the same investment strategies and criteria, although they are subject to different regulatory regimes (see “Business—Regulation” in our most recently filed Annual Report on Form 10-K, as well as in subsequent filings with the SEC, incorporated by reference herein). An investor’s return in MSCC will depend, in part, on the Taxable Subsidiaries’ and the Funds’ investment returns as they are wholly owned subsidiaries of MSCC.

The level of new portfolio investment activity will fluctuate from period to period based upon our view of the current economic fundamentals, our ability to identify new investment opportunities that meet our investment criteria, and our ability to consummate the identified opportunities. The level of new investment activity, and associated interest and fee income, will directly impact future investment income. In addition, the level of dividends paid by portfolio companies and the portion of our portfolio debt investments on non-accrual status will directly impact future investment income. While we intend to grow our portfolio and our investment income over the long term, our growth and our operating results may be more limited during depressed economic periods. However, we intend to appropriately manage our cost structure and liquidity position based on applicable economic conditions and our investment outlook. The level of realized gains or losses and unrealized appreciation or depreciation on our investments will also fluctuate depending upon portfolio activity, economic conditions and the performance of our individual portfolio companies. The changes in realized gains and losses and unrealized appreciation or depreciation could have a material impact on our operating results.

Because we are internally managed, we do not pay any external investment advisory fees, but instead directly incur the operating costs associated with employing investment and portfolio management professionals. We believe that our internally managed structure provides us with a better alignment of interests between our management team and our employees and our shareholders and a beneficial operating expense structure when compared to other publicly traded and privately held investment firms which are externally managed, and our internally managed structure allows us the opportunity to leverage our non-interest operating expenses as we grow our investment portfolio.

Through the External Investment Manager, we serve as the sole investment adviser and administrator to MSC Income pursuant to an Investment Advisory and Administrative Services Agreement entered into between the External Investment Manager and MSC Income (the “Advisory Agreement”). Under the Advisory Agreement, the External Investment Manager earns a 1.75% annual base management fee and a 20% incentive fee on MSC Income’s pre-investment fee net investment income above a specified hurdle rate, as well as an incentive fee on capital gains, in exchange for providing advisory services to MSC Income.

Additionally, the External Investment Manager has entered into an Investment Management Agreement with MS Private Loan Fund I, LP, a private investment fund with a strategy to co-invest with Main Street in Private Loan portfolio investments (the “Private Loan Fund”), pursuant to which the External Investment Manager provides investment advisory and management services to the Private Loan Fund in exchange for an asset-based fee and certain incentive fees.

We have entered into an agreement with the External Investment Manager to share employees in connection with its asset management business generally, and specifically for its relationship with MSC Income and its other clients. Through this agreement, we share employees with the External Investment Manager, including their related infrastructure, business relationships, management expertise and capital raising capabilities, and we allocate the related expenses to the External Investment Manager pursuant to the sharing agreement.

We have received an exemptive order from the SEC permitting co-investments among us, MSC Income and other funds and clients advised by the External Investment Manager in certain negotiated transactions where co-investing would otherwise be prohibited under the 1940 Act. We have made co-investments with, and in the future intend to continue to make co-investments with MSC Income, the Private Loan Fund and other clients advised by the External Investment Manager, in accordance with the conditions of the order. The order requires, among other things, that we and the External Investment Manager consider whether each such investment opportunity is appropriate for us and the External Investment Manager’s advised clients, as applicable, and if it is appropriate, to propose an allocation of the investment opportunity between such parties. Because the External Investment Manager may receive performance-

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based fee compensation from funds and clients advised by the External Investment Manager, this may provide the Company and the External Investment Manager an incentive to allocate opportunities to other participating funds and clients instead of us. However, both we and the External Investment Manager have policies and procedures in place to manage this conflict, including oversight by the independent members of our Board of Directors.

You should be aware that investments in our portfolio companies carry a number of risks including, but not limited to, investing in companies which may have limited operating histories and financial resources and other risks common to investing in below investment grade debt and equity investments in private, smaller companies. Please see “Risk Factors—Risks Related to Our Investments” in our most recently filed Annual Report on Form 10-K, as well as in subsequent filings with the SEC, for a more complete discussion of the risks involved with investing in our portfolio companies.

Our principal executive offices are located at 1300 Post Oak Boulevard, 8^th Floor, Houston, Texas 77056, and our telephone number is (713) 350-6000. We maintain a website at http://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.

Risks Associated with Our Business

Our business is subject to numerous risks, as described in the section titled “Risk Factors” in 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 documents that are incorporated by reference into this prospectus, including the section titled “Risk Factors” included in our most recent Annual Report on Form 10-K, as well as in subsequent filings with the SEC.

Dividend Reinvestment and Direct Stock Purchase Plan

We have adopted a dividend reinvestment and direct stock purchase plan (the “Plan”). The Plan primarily consists of a dividend reinvestment feature and a direct stock purchase feature. The direct stock purchase feature of the Plan is designed to provide new investors and existing holders of our common stock with a convenient and economical method to purchase shares of our common stock and is described in more detail in a separate prospectus supplement. The dividend reinvestment feature of the Plan, or the dividend reinvestment plan, provides for the reinvestment of dividends on behalf of our registered stockholders who hold their shares with American Stock Transfer & Trust Company, LLC, the plan administrator and our transfer agent and registrar, or certain brokerage firms that have elected to participate in our dividend reinvestment plan, unless a stockholder has elected to receive dividends in cash. For more information, see “Dividend Reinvestment and Direct Stock Purchase Plan.”

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The Offering

We may offer, from time to time in one or more offerings or series, our securities under this prospectus, together with the applicable prospectus supplement and any related free writing prospectus, at prices and on terms to be determined at the time of any offering. This prospectus provides you with a general description of the securities we may offer. Each time we offer a type or series of securities under this prospectus, we will provide a prospectus supplement that will describe the specific amounts, prices, and other important terms of the securities. The applicable prospectus supplement and any related free writing prospectus that we may authorize to be provided to you may also add, update, or change information contained in this prospectus or in documents we have incorporated by reference.

Our securities may be offered directly to one or more purchasers by us or through agents designated from time to time by us, or to or through underwriters or dealers. The prospectus supplement relating to the offering will disclose the terms of the offering, including the name or names of any agents or underwriters involved in the sale of our securities by us, the purchase price, and any 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.”

Set forth below is additional information regarding the offering of our securities:

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FEES AND EXPENSES

The following table is intended to assist you in understanding the costs and expenses that an investor in this offering will bear directly or indirectly. We caution you that some of the percentages indicated in the table below are estimates and may vary. Except where the context suggests otherwise, whenever this prospectus contains a reference to fees or expenses paid by “you,” “us” or “Main Street,” or that “we” will pay fees or expenses, stockholders will indirectly bear such fees or expenses as investors in us.

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Example

The following example demonstrates the projected dollar amount of total cumulative expenses that would be incurred over various periods with respect to a hypothetical investment in our common stock. In calculating the following expense amounts, we have assumed we would have no additional leverage and that our annual operating expenses would remain at the levels set forth in the table above. In the event that shares to which this prospectus relates are sold to or through underwriters, a corresponding prospectus supplement will restate this example to reflect the applicable sales load.

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The example and the expenses in the table above should not be considered a representation of our future expenses, and actual expenses may be greater or less than those shown. While the example assumes, as required by the SEC, a 5.0% annual return, our performance will vary and may result in a return greater or less than 5.0%. In addition, while the example assumes reinvestment of all dividends at net asset value, participants in our dividend reinvestment plan will receive a number of shares of our common stock, determined by dividing the total dollar amount of the dividend payable to a participant by (i) the market price per share of our common stock at the close of trading on a valuation date determined by our Board of Directors for each dividend in the event that we use newly issued shares to satisfy the share requirements of the dividend reinvestment plan or (ii) the average purchase price of all shares of common stock purchased by the plan administrator in the event that shares are purchased in the open market to satisfy the share requirements of the dividend reinvestment plan, which may be at, above or below net asset value. See “Dividend Reinvestment and Direct Stock Purchase Plan” for additional information regarding our dividend reinvestment plan.

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FINANCIAL HIGHLIGHTS

The financial data as of and for each of the ten years ended December 31, 2021 is set forth in “Note F – Financial Highlights” in the notes to our audited consolidated financial statements included in our Annual Report on Form 10-K for the fiscal year ended December 31, 2021 and incorporated by reference into this prospectus in its entirety. The financial data has been audited by Grant Thornton LLP, an independent registered public accounting firm whose reports thereon are incorporated by reference in this prospectus. A copy of our Annual Report on Form 10-K filed with the SEC may be obtained from www.sec.gov or upon request. You should read these financial highlights in conjunction with our consolidated financial statements and notes thereto and “Management’s Discussion and Analysis of Financial Condition and Results of Operations” incorporated by reference into this prospectus, any documents incorporated by reference in this prospectus or the accompanying prospectus supplement, or our Annual Reports on Form 10-K filed with the SEC.

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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:

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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:

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, including to repay or repurchase outstanding indebtedness, in which case the interest rate and maturity date of which will be described in a prospectus supplement or freewriting prospectus.

We anticipate that substantially all of the net proceeds of an offering of securities pursuant to this prospectus and its related prospectus supplement will be used for the above purposes within three months of any such offering, depending on the availability of appropriate investment opportunities consistent with our investment objective, but no longer than within six months of any such offerings.

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 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.

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On March 1, 2022, the last sale price of our common stock on the NYSE was $42.75 per share, and there were approximately 360 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, 2021 (the last date prior to the date of this prospectus on which we determined our net asset value per share) was $25.29, and the premium of the March 1, 2022 closing price of our common stock was 77% 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 initial public offering 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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MANAGEMENT’S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS OF OPERATIONS

The information contained under the caption “Management’s Discussion and Analysis of Financial Condition and Results of Operations” of our most recent Annual Report on Form 10-K is incorporated by reference herein.

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PORTFOLIO COMPANIES

The following table sets forth certain unaudited information as of December 31, 2021 (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, 2021, 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.

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