Use these links to rapidly review the document
TABLE OF CONTENTS
TABLE OF CONTENTS
INDEX TO FINANCIAL STATEMENTS

Table of Contents

Filed Pursuant to Rule 497
Registration Statement No. 333-173674

PROSPECTUS SUPPLEMENT
(to Prospectus dated May 25, 2012)

3,750,000 Shares

Main Street Capital Corporation

Common Stock

           We are offering for sale 3,750,000 shares of our common stock.

           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 $25 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies.

           The LMM securities in which we invest 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.

           Our common stock is listed on the New York Stock Exchange under the symbol "MAIN." On June 20, 2012, the last reported sale price of our common stock on the New York Stock Exchange was $23.72 per share.

           Shares of closed-end investment companies such as ours 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. In this regard, on June 14, 2012, our common stockholders voted to allow us to issue common stock at a price below net asset value per share for a period of one year ending on June 13, 2013. Our stockholders did not specify a maximum discount below net asset value at which we are able to issue our common stock, although the number of shares sold in each offering may not exceed 25% of our outstanding common stock immediately prior to such sale. In addition, we cannot issue shares of our common stock below net asset value unless our Board of Directors determines that it would be in our and our stockholders' best interests to do so. 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, 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" in the accompanying prospectus.

           Investing in our common stock involves a high degree of risk, and should be considered highly speculative. See "Risk Factors" beginning on page 13 of the accompanying prospectus to read about factors you should consider, including the risk of leverage and dilution, 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. Please read this prospectus supplement and the accompanying prospectus before investing and keep them for future reference. 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, Suite 800, 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 562,500 shares of common stock at the public offering price, less the underwriting discount, within 30 days from the date of this prospectus supplement solely to cover any over-allotments. If the over-allotment option is exercised in full, the total public offering price will be $97,031,250, the total underwriting discount (4.00%) will be $3,881,250, and the total proceeds to us, before deducting estimated expenses payable by us of $200,000, will be $93,150,000.

           The Securities and Exchange Commission has not 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 June 26, 2012.

RAYMOND JAMES

JANNEY MONTGOMERY SCOTT

      LADENBURG THALMANN & CO. INC.

                                                                       SANDERS MORRIS HARRIS

The date of this prospectus supplement is June 21, 2012

Table of Contents

TABLE OF CONTENTS

i

Table of Contents

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. The second part is the accompanying prospectus, which provides more information about the common stock we may offer from time to time. To the extent the information contained in this prospectus supplement differs from the information contained in the accompanying prospectus, the information in this prospectus supplement shall control.

        You should rely only on the information contained in this prospectus supplement and the accompanying prospectus. Neither we nor the underwriters have authorized any other person to provide you with different information from that contained in this prospectus supplement or the accompanying prospectus. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement and the accompanying prospectus 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. The information contained in this prospectus supplement and the accompanying prospectus is complete and accurate only as of their respective dates, regardless of the time of their delivery or sale of our common stock. This prospectus supplement supersedes the accompanying prospectus to the extent it contains information different from or additional to the information in that prospectus.

Forward-Looking Statements

        Information contained in this prospectus supplement and the accompanying prospectus 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 certain other factors noted throughout this prospectus supplement and the accompanying prospectus 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. We note that the safe harbor for forward-looking statements provided by the Private Securities Litigation Reform Act of 1995 does not apply to statements made in this prospectus supplement or the accompanying prospectus.

ii

Table of Contents

S-1

Table of Contents

S-2

Table of Contents

S-3

Table of Contents

S-4

Table of Contents

S-5

Table of Contents

S-6

Table of Contents

S-7

Table of Contents

S-8

Table of Contents

S-9

Table of Contents

USE OF PROCEEDS

        The net proceeds from the sale of the 3,750,000 shares of common stock in this offering are $80,800,000, and $92,950,000 if the underwriter's over-allotment option 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 $277.5 million credit facility. However, through re-borrowing of the initial repayments under our $277.5 million 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 Common Stock—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 the accompanying prospectus.

        At June 19, 2012, we had approximately $174 million outstanding under our $277.5 million credit facility. Our credit facility matures on September 20, 2014, unless extended, and bears interest, at our election, on a per annum basis equal to (i) the applicable LIBOR rate plus 2.50% or (ii) the applicable base rate plus 1.50%. Amounts repaid under our $277.5 million credit facility will remain available for future borrowings. As of March 31, 2012, the interest rate on our $277.5 million credit facility was 2.8%.

        An affiliate of BB&T Capital Markets, an underwriter in this offering, acts as agent and a lender under our $277.5 million 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 affiliate therefore may receive a portion of the proceeds from this offering through the repayment of those borrowings. See "Underwriting—Conflicts of Interest" below.

S-10

Table of Contents

CAPITALIZATION

        The following table sets forth our capitalization:

•

on an actual basis as of March 31, 2012; and



•

on an as-adjusted basis giving effect to the sale of 3,750,000 shares of our common stock in this offering at the public offering price of $22.50 per share, less estimated underwriting discounts and offering expenses payable by us.

        This table should be read in conjunction with "Interim Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Interim Financial Statements" in the accompanying prospectus.

S-11

Table of Contents

UNDERWRITING

        Under the terms and subject to the conditions contained in an underwriting agreement dated June 21, 2012, the underwriters named below, for whom Raymond James & Associates, Inc. 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:

        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' over-allotment option described below) if any such shares are taken. We have agreed to indemnify the 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."

Over-Allotment Option

        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 562,500 additional shares of common stock at the public offering price set forth on the cover page hereof, less the underwriting discount. The underwriters may exercise this option solely for the purpose of covering over-allotments, if any, made in connection with the offering of the shares of common stock offered hereby. 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, subject to certain exceptions, not to issue, sell, offer to sell, contract or agree to sell, hypothecate, pledge, transfer, grant any option to purchase, establish an open put equivalent position or otherwise dispose of or agree to dispose of directly or indirectly, any shares of our common stock, or any securities convertible into or exercisable or exchangeable for any shares of our common stock or any right to acquire shares of our common stock, for 30 days from the date of this prospectus supplement, subject to extension upon material announcements or earnings releases. The representative, at

S-12

Table of Contents

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 $0.54 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 562,500 additional shares from us.

        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.

S-13

Table of Contents

        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.

Conflicts of Interest

        An affiliate of BB&T Capital Markets, an underwriter in this offering, acts as agent and a lender under our $277.5 million credit facility. Certain of the net proceeds from the sale of our common stock, not including underwriting compensation, may be paid to such affiliate of BB&T Capital Markets in connection with the repayment of debt owed under our $277.5 million credit facility. As a result, BB&T Capital Markets and/or its affiliate 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 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 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: Raymond James & Associates, Inc., 880 Carillon Parkway, St. Petersburg, Florida 33716; BB&T Capital Markets, 901 East Byrd Street, Suite 410, Richmond, Virginia 23219; Robert W. Baird & Co. Incorporated, 777 East Wisconsin Avenue, Milwaukee, Wisconsin 53202; Janney Montgomery Scott LLC, 1801 Market Street, Philadelphia, Pennsylvania 19103; Ladenburg Thalmann & Co. Inc., 520 Madison Avenue, 9th Floor, New York, New York 10022; Sanders Morris Harris Inc., 600 Travis, Suite 5800, Houston, Texas 77002.

LEGAL MATTERS

        Certain legal matters regarding the shares of common stock offered hereby will be passed upon for us by Sutherland Asbill & Brennan LLP, Washington D.C., and certain legal matters in connection with this offering will be passed upon for the underwriters by Bass, Berry & Sims PLC, Memphis, Tennessee.

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

        The consolidated financial statements, Schedule 12-14 and the schedule of Senior Securities of Main Street Capital Corporation, included in the accompanying prospectus have been so included in reliance upon the reports of Grant Thornton LLP, independent registered public

S-14

Table of Contents

accountants, upon the authority of said firm as experts in giving said reports. Grant Thornton LLP's principal business address is 333 Clay Street, 2700 Three Allen Center, Houston, Texas 77002.

AVAILABLE INFORMATION

        We have filed with the SEC a registration statement on Form N-2, together with all amendments and related exhibits, under the Securities Act, with respect to our shares of common stock offered by this prospectus supplement. The registration statement contains additional information about us and our shares of common stock being offered by this prospectus supplement.

        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. You may inspect and copy these reports, proxy statements and other information, as well as the registration statement and related exhibits and schedules, at the Public Reference Room of the SEC at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the Public Reference Room by calling the SEC at 1-800-SEC-0330. 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 on the SEC's website at www.sec.gov. Copies of these reports, proxy and information statements and other information may be obtained, after paying a duplicating fee, by electronic request at the following e-mail address: publicinfo@sec.gov, or by writing the SEC's Public Reference Section, 100 F Street, N.E., Washington, D.C. 20549.

S-15

Table of Contents

PROSPECTUS

$500,000,000

Main Street Capital Corporation

Common Stock

           We may offer, from time to time, up to $500,000,000 of our common stock, $0.01 par value per share, in one or more offerings. Our common stock 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. On June 15, 2011, our common stockholders voted to allow us to issue common stock at a price below net asset value per share for a period of one year ending on June 14, 2012. Our stockholders did not specify a maximum discount below net asset value at which we are able to issue our common stock; however, we cannot issue shares of our common stock below net asset value unless our Board of Directors determines that it would be in our and our stockholders' best interests to do so. We are seeking approval of a similar proposal from our stockholders at our 2012 annual stockholders meeting to be held on June 14, 2012. 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, 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 this prospectus and the applicable prospectus supplement carefully before you invest in our common stock.

           Our common stock may be offered directly to one or more purchasers 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 common stock, 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 may not sell any of our common stock through agents, underwriters or dealers without delivery of a prospectus supplement describing the method and terms of the offering of such common stock, which must be delivered to each purchaser at, or prior to, the earlier of delivery of a confirmation of sale or delivery of the shares of common stock.

           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 $25 million. Our Middle Market investments are made in businesses that are generally larger in size than our LMM portfolio companies.

           The LMM securities in which we invest 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.

           Our common stock is listed on the New York Stock Exchange under the symbol "MAIN." On May 22, 2012, the last reported sale price of our common stock on the New York Stock Exchange was $23.38 per share.

        Investing in our common stock involves a high degree of risk, and should be considered highly speculative. See "Risk Factors" beginning on page 13 to read about factors you should consider, including the risk of leverage and dilution, before investing in our common stock.

           This prospectus and the accompanying prospectus supplement contain important information about us that a prospective investor should know before investing in our common stock. Please read this prospectus and the accompanying prospectus supplement before investing and keep them for future reference. We file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission. This information is available free of charge by contacting us at 1300 Post Oak Boulevard, Suite 800, 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 Securities and Exchange Commission also maintains a website at www.sec.gov that contains such information.

           Neither the Securities and Exchange Commission nor any state securities commission 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.

The date of this prospectus is May 25, 2012

Table of Contents

TABLE OF CONTENTS

        This prospectus is part of a registration statement that we have filed with the Securities and Exchange Commission, or SEC, using the "shelf" registration process. Under the shelf registration process, we may offer, from time to time, up to $500,000,000 of our common stock on terms to be determined at the time of the offering. This prospectus provides you with a general description of the common stock that we may offer. Each time we use this prospectus to offer common stock, we will provide a prospectus supplement that will contain specific information about the terms of that offering. The prospectus supplement may also add, update or change information contained in this prospectus. To the extent required by law, we will amend or supplement the information contained in this prospectus and any accompanying prospectus supplement to reflect any material changes to such information subsequent to the date of the prospectus and any accompanying prospectus supplement and prior to the completion of any offering pursuant to the prospectus and any accompanying prospectus supplement. Please carefully read this prospectus and any accompanying prospectus supplement together with the additional information described under "Available Information" and "Risk Factors" before you make an investment decision.

        No dealer, salesperson or other person is authorized to give any information or to represent anything not contained in this prospectus or any accompanying supplement to this prospectus. You must not rely on any unauthorized information or representations not contained in this prospectus or any accompanying prospectus supplement as if we had authorized it. This prospectus and any accompanying prospectus supplement do not constitute an offer to sell or a solicitation of any offer to buy any security other than the registered securities to which they relate, nor do they constitute an offer to sell or a solicitation of an offer to buy any securities in any jurisdiction to any person to whom it is unlawful to make such an offer or solicitation in such jurisdiction. The information contained in this prospectus and any accompanying prospectus supplement is accurate as of the dates on their covers.

Table of Contents

1

Table of Contents

2

Table of Contents

3

Table of Contents

4

Table of Contents

5

Table of Contents

6

Table of Contents

7

Table of Contents

8

Table of Contents

9

Table of Contents

10

Table of Contents

11

Table of Contents

12

Table of Contents

RISK FACTORS

        Investing in our common stock involves a number of significant risks. In addition to the other information contained in this prospectus and any accompanying prospectus supplement, you should consider carefully the following information before making an investment in our common stock. The risks set out below are not the only risks we face. Additional risks and uncertainties not presently known to us or not presently deemed material by us might also impair our operations and performance. If any of the following events occur, our business, financial condition and results of operations could be materially and adversely affected. In such case, our net asset value and the trading price of our common stock could decline, and you may lose all or part of your investment.

Risks Relating to Economic Conditions

Deterioration in the economy and financial markets increases the likelihood of adverse effects on our financial position and results of operations. Such economic adversity could impair our portfolio companies' financial positions and operating results and affect the industries in which we invest, which could, in turn, harm our operating results.

        The broader economic fundamentals of the United States economy remain uncertain. Unemployment levels remain elevated and other economic fundamentals remain depressed. In the event that the United States economic performance contracts, it is likely that the financial results of small- to mid-sized companies, like those in which we invest, could experience deterioration or limited growth, which could ultimately lead to difficulty in meeting their debt service requirements and an increase in defaults. Consequently, we can provide no assurance that the performance of certain of our portfolio companies will not be negatively impacted by economic or other conditions, which could also have a negative impact on our future results.

        Although we have been able to secure access to additional liquidity, including our $235 million credit facility, periodic follow-on equity offerings, and the increase in available leverage through the SBIC program as part of the American Recovery and Reinvestment Act of 2009 enacted in February 2009 (the "Stimulus Bill"), the potential for volatility in the debt and equity capital markets provides no assurance that debt or equity capital will be available to us in the future on favorable terms, or at all.

Risks Relating to Our Business and Structure

Our investment portfolio is and will continue to be recorded at fair value, with our Board of Directors having final responsibility for overseeing, reviewing and approving, in good faith, our determination of fair value and, as a result, there is and will continue to be uncertainty as to the value of our portfolio investments.

        Under the 1940 Act, we are required to carry our portfolio investments at market value or, if there is no readily available market value, at fair value as determined by us with our Board of Directors having final responsibility for overseeing, reviewing and approving, in good faith, our determination of fair value. Typically, there is not a public market for the securities of the privately held LMM companies in which we have invested and will generally continue to invest. As a result, we value these securities quarterly at fair value based on inputs from management, a nationally recognized independent advisor (on a rotational basis) and our audit committee with the oversight, review and approval of our Board of Directors. In addition, the market for Middle Market portfolio investments is generally not a liquid market, and therefore, we primarily use observable inputs to determine the fair value of these investments quarterly through obtaining third party quotes and other independent pricing, which are reviewed by our audit committee with the oversight, review and approval of our Board of Directors. See "Business—Determination of Net Asset Value and Portfolio Valuation Process" for a more detailed description of our valuation process.

13

Table of Contents

        The determination of fair value and consequently, the amount of unrealized gains and losses in our portfolio, are to a certain degree, subjective and dependent on a valuation process approved by our Board of Directors. Certain factors that may be considered in determining the fair value of our investments include external events, such as private mergers, sales and acquisitions involving comparable companies. Because such valuations, and particularly valuations of private securities and private companies, are inherently uncertain, may fluctuate over short periods of time and may be based on estimates, our determinations of fair value may differ materially from the values that would have been used if a ready market for these securities existed. Due to this uncertainty, our fair value determinations may cause our net asset value on a given date to materially understate or overstate the value that we may ultimately realize on one or more of our investments. As a result, investors purchasing our common stock based on an overstated net asset value would pay a higher price than the value of our investments might warrant. Conversely, investors selling shares during a period in which the net asset value understates the value of our investments will receive a lower price for their shares than the value of our investments might warrant.

Our financial condition and results of operations depends on our ability to effectively manage and deploy capital.

        Our ability to achieve our investment objective of maximizing 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, depends on our ability to effectively manage and deploy capital, which depends, in turn, on our investment team's ability to identify, evaluate and monitor, and our ability to finance and invest in, companies that meet our investment criteria.

        Accomplishing our investment objective on a cost-effective basis is largely a function of our investment team's handling of the investment process, its ability to provide competent, attentive and efficient services and our access to investments offering acceptable terms. In addition to monitoring the performance of our existing investments, members of our investment team are also called upon, from time to time, to provide managerial assistance to some of our portfolio companies. These demands on their time may distract them or slow the rate of investment.

        Even if we are able to grow and build upon our investment operations, any failure to manage our growth effectively could have a material adverse effect on our business, financial condition, results of operations and prospects. The results of our operations will depend on many factors, including the availability of opportunities for investment, readily accessible short and long-term funding alternatives in the financial markets and economic conditions. Furthermore, if we cannot successfully operate our business or implement our investment policies and strategies as described herein, it could negatively impact our ability to pay dividends.

We may face increasing competition for investment opportunities.

        We compete for investments with other investment funds (including private equity funds, mezzanine funds, BDCs, and SBICs), as well as traditional financial services companies such as commercial banks and other sources of funding. Many of our competitors are substantially larger and have considerably greater financial, technical and marketing resources than we do. For example, some competitors may have a lower cost of capital and access to funding sources that are not available to us. In addition, some of our competitors may have higher risk tolerances or different risk assessments than we have. These characteristics could allow our competitors to consider a wider variety of investments, establish more relationships and offer better pricing and more flexible structuring than we are able to do. We may lose investment opportunities if we do not match our competitors' pricing, terms and structure. If we are forced to match our competitors' pricing, terms and structure, we may not be able to achieve acceptable returns on our investments or may bear substantial risk of capital loss. A

14

Table of Contents

significant part of our competitive advantage stems from the fact that the market for investments in LMM companies is underserved by traditional commercial banks and other financing sources. A significant increase in the number and/or the size of our competitors in this target market could force us to accept less attractive investment terms. Furthermore, many of our competitors have greater experience operating under, or are not subject to, the regulatory restrictions that the 1940 Act imposes on us as a BDC.

We are dependent upon our key investment personnel for our future success.

        We depend on the members of our investment team, particularly Vincent D. Foster, Todd A. Reppert, Rodger A. Stout, Curtis L. Hartman, Dwayne L. Hyzak and David L. Magdol, for the identification, review, final selection, structuring, closing and monitoring of our investments. These employees have significant investment expertise and relationships that we rely on to implement our business plan. Although we have entered into a non-compete agreement with Mr. Foster, we have no guarantee that he or any other employees will remain employed with us. If we lose the services of these individuals, we may not be able to operate our business as we expect, and our ability to compete could be harmed, which could cause our operating results to suffer.

Our success depends on attracting and retaining qualified personnel in a competitive environment.

        Our growth will require that we retain new investment and administrative personnel in a competitive market. Our ability to attract and retain personnel with the requisite credentials, experience and skills depends on several factors including, but not limited to, our ability to offer competitive wages, benefits and professional growth opportunities. Many of the entities, including investment funds (such as private equity funds and mezzanine funds) and traditional financial services companies, with which we compete for experienced personnel have greater resources than we have.

        The competitive environment for qualified personnel may require us to take certain measures to ensure that we are able to attract and retain experienced personnel. Such measures may include increasing the attractiveness of our overall compensation packages, altering the structure of our compensation packages through the use of additional forms of compensation, or other steps. The inability to attract and retain experienced personnel would have a material adverse effect on our business.

Our business model depends to a significant extent upon strong referral relationships, and our inability to maintain or develop these relationships, as well as the failure of these relationships to generate investment opportunities, could adversely affect our business.

        We expect that members of our management team will maintain their relationships with intermediaries, financial institutions, investment bankers, commercial bankers, financial advisors, attorneys, accountants, consultants and other individuals within our network, and we will rely to a significant extent upon these relationships to provide us with potential investment opportunities. If our management team fails to maintain its existing relationships or develop new relationships with sources of investment opportunities, we will not be able to grow our investment portfolio. In addition, individuals with whom members of our management team have relationships are not obligated to provide us with investment opportunities, and, therefore, there is no assurance that such relationships will generate investment opportunities for us.

15

Table of Contents

Regulations governing our operation as a BDC will affect our ability to, and the way in which we, raise additional capital.

        Our business will require capital to operate and grow. We may acquire such additional capital from the following sources:

        Senior Securities.    We may issue debt securities or preferred stock and/or borrow money from banks or other financial institutions, which we refer to collectively as senior securities. As a result of issuing senior securities, we will be exposed to additional risks, including the following:

•

Under the provisions of the 1940 Act, we are permitted, as a BDC, to issue senior securities only in amounts such that our asset coverage, as defined in the 1940 Act, equals at least 200% immediately after each issuance of senior securities. If the value of our assets declines, we may be unable to satisfy this test. If that happens, we will be prohibited from issuing debt securities or preferred stock and/or borrowing money from banks or other financial institutions and may not be permitted to declare a dividend or make any distribution to stockholders or repurchase shares until such time as we satisfy this test.



•

Any amounts that we use to service our debt or make payments on preferred stock will not be available for dividends to our common stockholders.



•

It is likely that any senior securities or other indebtedness we issue will be governed by an indenture or other instrument containing covenants restricting our operating flexibility. Additionally, some of these securities or other indebtedness may be rated by rating agencies, and in obtaining a rating for such securities and other indebtedness, we may be required to abide by operating and investment guidelines that further restrict operating and financial flexibility.



•

We and, indirectly, our stockholders will bear the cost of issuing and servicing such securities and other indebtedness.



•

Preferred stock or any convertible or exchangeable securities that we issue in the future may have rights, preferences and privileges more favorable than those of our common stock, including separate voting rights and could delay or prevent a transaction or a change in control to the detriment of the holders of our common stock.

        Additional Common Stock.    We are not generally able to issue and sell our common stock at a price below net asset value per share. We may, however, sell our common stock, warrants, options or rights to acquire our common stock, at a price below the current net asset value of the common stock if our Board of Directors determines that such sale is in the best interests of our stockholders, and our stockholders approve such sale. See "—Stockholders may incur dilution if we sell shares of our common stock in one or more offerings at prices below the then current net asset value per share of our common stock or issue securities to subscribe to, convert to or purchase shares of our common stock" for a discussion of proposals approved by our stockholders that permit us to issue shares of our common stock below net asset value. We may also make rights offerings to our stockholders at prices per share less than the net asset value per share, subject to applicable requirements of the 1940 Act. If we raise additional funds by issuing more common stock or senior securities convertible into, or exchangeable for, our common stock, the percentage ownership of our stockholders at that time would decrease, and they may experience dilution. Moreover, we can offer no assurance that we will be able to issue and sell additional equity securities in the future, on favorable terms or at all.

The Funds are licensed by the SBA, and therefore subject to SBA regulations.

        MSMF and MSC II, our wholly owned subsidiaries, are licensed to act as SBICs and are regulated by the SBA. The SBA also places certain limitations on the financing terms of investments by SBICs in portfolio companies and prohibits SBICs from providing funds for certain purposes or to businesses in

16

Table of Contents

a few prohibited industries. Compliance with SBA requirements may cause the Funds to forego attractive investment opportunities that are not permitted under SBA regulations.

        Further, the SBA regulations require that a licensed SBIC be periodically examined and audited by the SBA to determine its compliance with the relevant SBA regulations. The SBA prohibits, without prior SBA approval, a "change of control" of an SBIC or transfers that would result in any person (or a group of persons acting in concert) owning 10% or more of a class of capital stock of a licensed SBIC. If the Funds fail to comply with applicable SBIC regulations, the SBA could, depending on the severity of the violation, limit or prohibit their use of debentures, declare outstanding debentures immediately due and payable, and/or limit them from making new investments. In addition, the SBA can revoke or suspend a license for willful or repeated violation of, or willful or repeated failure to observe, any provision of the Small Business Investment Act of 1958 or any rule or regulation promulgated thereunder. Such actions by the SBA would, in turn, negatively affect us.

Because we borrow money, the potential for gain or loss on amounts invested in us is magnified and may increase the risk of investing in us.

        Borrowings, also known as leverage, magnify the potential for gain or loss on invested equity capital. As we use leverage to partially finance our investments, you will experience increased risks of investing in our common stock. We, through the Funds, issue debt securities guaranteed by the SBA and sold in the capital markets. As a result of its guarantee of the debt securities, the SBA has fixed dollar claims on the assets of the Funds that are superior to the claims of our common stockholders. We may also borrow from banks and other lenders, including under our $235 million credit facility (the "Credit Facility"). See "Management's Discussion and Analysis of Financial Condition and Results of Operations—Capital Resources" for a discussion regarding our Credit Facility. If the value of our assets increases, then leveraging would cause the net asset value attributable to our common stock to increase more sharply than it would have had we not leveraged. Conversely, if the value of our assets decreases, leveraging would cause net asset value to decline more sharply than it otherwise would have had we not leveraged our business. Similarly, any increase in our income in excess of interest payable on the borrowed funds would cause our net investment income to increase more than it would without the leverage, while any decrease in our income would cause net investment income to decline more sharply than it would have had we not borrowed. Such a decline could negatively affect our ability to pay common stock dividends. Leverage is generally considered a speculative investment technique.

        As of March 31, 2012, we, through the Funds, had $220 million of outstanding indebtedness guaranteed by the SBA, which had a weighted average annualized interest cost of approximately 5.1% (exclusive of deferred financing costs). The debentures guaranteed by the SBA have a maturity of ten years, with a current weighted average remaining maturity of 6.4 years as of March 31, 2012, and require semi-annual payments of interest. We will need to generate sufficient cash flow to make required interest payments on the debentures. If we are unable to meet the financial obligations under the debentures, the SBA, as a creditor, will have a superior claim to the assets of the Funds over our stockholders in the event we liquidate or the SBA exercises its remedies under such debentures as the result of a default by us. In addition, as of March 31, 2012, we had $138 million outstanding under our Credit Facility. Borrowings under the Credit Facility bear interest, subject to our election, on a per annum basis equal to (i) the applicable LIBOR rate plus 2.50% or (ii) the applicable base rate plus 1.50%. Main Street pays unused commitment fees of 0.375% per annum on the average unused lender commitments under the Credit Facility. If we are unable to meet the financial obligations under the Credit Facility, the Credit Facility lending group will have a superior claim to the assets of MSCC and its subsidiaries (excluding the assets of the Funds) over our stockholders in the event we liquidate or the lending group exercises its remedies under the Credit Facility as the result of a default by us.

17

Table of Contents

        Illustration.    The following table illustrates the effect of leverage on returns from an investment in our common stock assuming various annual returns, net of expenses. The calculations in the table below are hypothetical and actual returns may be higher or lower than those appearing below.

Assumed Return on Our Portfolio(1)
(net of expenses)

        Our ability to achieve our investment objective may depend in part on our ability to access additional leverage on favorable terms by issuing debentures guaranteed by the SBA, through the Funds, or by borrowing from banks or insurance companies, and there can be no assurance that such additional leverage can in fact be achieved.

We may experience fluctuations in our quarterly results.

        We could experience fluctuations in our quarterly operating results due to a number of factors, including our ability or inability to make investments in companies that meet our investment criteria, the interest rate payable on the debt securities we acquire, the level of portfolio dividend and fee income, the level of our expenses, variations in and the timing of the recognition of realized and unrealized gains or losses, the degree to which we encounter competition in our markets and general economic conditions. As a result of these factors, results for any period should not be relied upon as being indicative of performance in future periods.

Our Board of Directors may change our operating policies and strategies without prior notice or stockholder approval, the effects of which may be adverse.

        Our Board of Directors has the authority to modify or waive our current operating policies, investment criteria and strategies without prior notice and without stockholder approval. We cannot predict the effect any changes to our current operating policies, investment criteria and strategies would have on our business, net asset value, operating results and value of our stock. However, the effects might be adverse, which could negatively impact our ability to pay you dividends and cause you to lose all or part of your investment.

We will be subject to corporate-level income tax if we are unable to qualify as a RIC under Subchapter M of the Code.

        To maintain RIC tax treatment under the Code, we must meet the following annual distribution, income source and asset diversification requirements:

•

The annual distribution requirement for a RIC will be satisfied if we distribute to our stockholders on an annual basis at least 90% of our net ordinary income and realized net short-term capital gains in excess of realized net long-term capital losses, if any. Depending on the level of taxable income earned in a tax year, we may choose to carry forward taxable income in excess of current year distributions into the next tax year and pay a 4% excise tax on such income. Any such carryover taxable income must be distributed through a dividend declared prior to filing the final tax return related to the year which generated such taxable income. For

18

Table of Contents

more information regarding tax treatment, see "Material U.S. Federal Income Tax Considerations—Taxation as a Regulated Investment Company." Because we use debt financing, we are subject to certain asset coverage ratio requirements under the 1940 Act and are (and may in the future become) subject to certain financial covenants under loan and credit agreements that could, under certain circumstances, restrict us from making distributions necessary to satisfy the distribution requirement. If we are unable to obtain cash from other sources, we could fail to qualify for RIC tax treatment and thus become subject to corporate-level income tax.

•

The source income requirement will be satisfied if we obtain at least 90% of our income for each year from distributions, interest, gains from the sale of stock or securities or similar sources.



•

The asset diversification requirement will be satisfied if we meet certain asset diversification requirements at the end of each quarter of our taxable year. To satisfy this requirement, at least 50% of the value of our assets must consist of cash, cash equivalents, U.S. Government securities, securities of other RICs, and other acceptable securities; and no more than 25% of the value of our assets can be invested in the securities, other than U.S. government securities or securities of other RICs, of one issuer, of two or more issuers that are controlled, as determined under applicable Code rules, by us and that are engaged in the same or similar or related trades or businesses or of certain "qualified publicly traded partnerships."

        Failure to meet these requirements may result in our having to dispose of certain investments quickly in order to prevent the loss of RIC status. Because most of our investments will be in private companies, and therefore will be illiquid, any such dispositions could be made at disadvantageous prices and could result in substantial losses. Moreover, if we fail to maintain RIC tax treatment for any reason and are subject to corporate income tax, the resulting corporate taxes could substantially reduce our net assets, the amount of income available for distribution and the amount of our distributions.

We may not be able to pay you distributions, our distributions may not grow over time, and a portion of distributions paid to you may be a return of capital.

        We intend to pay monthly distributions to our stockholders out of assets legally available for distribution. We cannot assure you that we will achieve investment results that will allow us to pay a specified level of cash distributions, previously projected distributions for future periods, or year-to-year increases in cash distributions. Our ability to pay distributions might be adversely affected by, among other things, the impact of one or more of the risk factors described herein. In addition, the inability to satisfy the asset coverage test applicable to us as a BDC could limit our ability to pay distributions. All distributions will be paid at the discretion of our Board of Directors and will depend on our earnings, our financial condition, maintenance of our RIC status, compliance with applicable BDC regulations, each of the Funds' compliance with applicable SBIC regulations and such other factors as our Board of Directors may deem relevant from time to time. We cannot assure you that we will pay distributions to our stockholders in the future.

        When we make monthly distributions, we will be required to determine the extent to which such distributions are paid out of current or accumulated earnings, recognized capital gains or capital. To the extent there is a return of capital, investors will be required to reduce their basis in our stock for federal tax purposes, which will result in higher tax liability when the shares are sold, even if they have not increased in value or have lost value. In addition, any return of capital will be net of any sales load and offering expenses associated with sales of shares of our common stock. In the future, our distributions may include a return of capital.

19

Table of Contents

We may have difficulty paying our required distributions if we recognize income before or without receiving cash representing such income.

        We will include in income certain amounts that we have not yet received in cash, such as: (i) amortization of original issue discount, which may arise if we receive warrants in connection with the origination of a loan such that ascribing a value to the warrants creates original issue discount in the debt instrument or possibly in other circumstances; (ii) contractual payment-in-kind, or PIK, interest, which represents contractual interest added to the loan balance and due at the end of the loan term; (iii) contractual preferred dividends, which represents contractual dividends added to the preferred stock and due at the end of the preferred stock term; or (iv) amortization of market discount, which is associated with loans purchased in the secondary market at a discount to par value. Such amortization of original issue discounts, increases in loan balances as a result of contractual PIK arrangements, cumulative preferred dividends, or amortization of market discount will be included in income before we receive the corresponding cash payments. We also may be required to include in income certain other amounts before we receive such amounts in cash. Investments structured with these features may represent a higher level of credit risk compared to investments generating income which must be paid in cash on a current basis. For the three months ended March 31, 2012, (i) approximately 3.9% of our total investment income was attributable to PIK interest income, (ii) approximately 2.4% of our total investment income was attributable to amortization of original issue discount, (iii) approximately 2.0% of our total investment income was attributable to contractual preferred dividends, and (iv) approximately 0.1% of our total investment income was attributable to amortization of market discount on loans purchased in the secondary market at a discount.

        Since, in certain cases, we may recognize income before or without receiving cash representing such income, we may have difficulty meeting the annual distribution requirement necessary to maintain RIC tax treatment under the Code. Accordingly, we may have to sell some of our investments at times and/or at prices we would not consider advantageous, raise additional debt or equity capital or forgo new investment opportunities for this purpose. If we are not able to obtain cash from other sources, we may fail to qualify for RIC tax treatment and thus become subject to corporate-level income tax. For additional discussion regarding the tax implications of a RIC, please see "Material U.S. Federal Income Tax Considerations—Taxation as a Regulated Investment Company."

We may in the future choose to pay dividends in our own stock, in which case you may be required to pay tax in excess of the cash you receive.

        We may distribute taxable dividends that are payable in part in our stock. The IRS has issued private letter rulings providing that a dividend payable in stock or in cash at the election of the stockholders will be treated as a taxable dividend eligible for the dividends paid deduction provided that at least 20% of the total dividend is payable in cash and certain other requirements are satisfied. Taxable stockholders receiving such dividends will be required to include the full amount of the dividend as ordinary income (or as long-term capital gain to the extent such dividend is properly reported as a capital gain dividend) to the extent of our current and accumulated earnings and profits for United States federal income tax purposes. As a result, a U.S. stockholder may be required to pay tax with respect to such dividends in excess of any cash received. If a U.S. stockholder sells the stock it receives as a dividend in order to pay this tax, the sales proceeds may be less than the amount included in income with respect to the dividend, depending on the market price of our stock at the time of the sale. Furthermore, with respect to non-U.S. stockholders, we may be required to withhold U.S. tax with respect to such dividends, including in respect of all or a portion of such dividend that is payable in stock. In addition, if a significant number of our stockholders determine to sell shares of our stock in order to pay taxes owed on dividends, it may put downward pressure on the trading price of our stock.

20

Table of Contents

Each of the Funds, as an SBIC, may be unable to make distributions to us that will enable us to meet or maintain RIC status, which could result in the imposition of an entity-level tax.

        In order for us to continue to qualify for RIC tax treatment and to minimize corporate-level taxes, we will be required to distribute substantially all of our net ordinary income and net capital gain income, including income from certain of our subsidiaries, which includes the income from the Funds. We will be partially dependent on the Funds for cash distributions to enable us to meet the RIC distribution requirements. The Funds may be limited by the Small Business Investment Act of 1958, and SBIC regulations governing SBICs, from making certain distributions to us that may be necessary to enable us to maintain our status as a RIC. We may have to request a waiver of the SBA's restrictions for the Funds to make certain distributions to maintain our eligibility for RIC status. We cannot assure you that the SBA will grant such waiver and if the Funds are unable to obtain a waiver, compliance with the SBIC regulations may result in loss of RIC tax treatment and a consequent imposition of an entity-level tax on us.

Because we intend to distribute substantially all of our income to our stockholders to maintain our status as a RIC, we will continue to need additional capital to finance our growth, and regulations governing our operation as a BDC will affect our ability to, and the way in which we, raise additional capital and make distributions.

        In order to satisfy the requirements applicable to a RIC and to minimize corporate-level taxes, we intend to distribute to our stockholders substantially all of our net ordinary income and net capital gain income. We may carry forward excess undistributed taxable income into the next year, net of the 4% excise tax. Any such carryover taxable income must be distributed through a dividend declared prior to filing the final tax return related to the year which generated such taxable income. As a BDC, we generally are required to meet an asset coverage ratio, as defined in the 1940 Act, of at least 200% immediately after each issuance of senior securities. This requirement limits the amount that we may borrow and may prohibit us from making distributions. Because we will continue to need capital to grow our investment portfolio, this limitation may prevent us from incurring debt and require us to raise additional equity at a time when it may be disadvantageous to do so.

        While we expect to be able to borrow and to issue additional debt and equity securities, we cannot assure you that debt and equity financing will be available to us on favorable terms, or at all. In addition, as a BDC, we generally are not permitted to issue equity securities priced below net asset value without stockholder approval. If additional funds are not available to us, we could be forced to curtail or cease new investment activities, and our net asset value could decline.

Stockholders may incur dilution if we sell shares of our common stock in one or more offerings at prices below the then current net asset value per share of our common stock or issue securities to subscribe to, convert to or purchase shares of our common stock.

        The 1940 Act prohibits us from selling shares of our common stock at a price below the current net asset value per share of such stock, with certain exceptions. One such exception is prior stockholder approval of issuances below net asset value provided that our Board of Directors makes certain determinations. At our 2011 annual meeting of stockholders, our stockholders approved a proposal that authorizes us to sell shares of our common stock below the then current net asset value per share of our common stock in one or more offerings for a period of one year ending on June 14, 2012. Continued access to this exception will require approval of similar proposals at future stockholder meetings. We are seeking such approval for the next year at our 2012 annual stockholders meeting to be held on June 14, 2012. At our 2008 annual meeting of stockholders, our stockholders approved a proposal to authorize us to issue securities to subscribe to, convert to, or purchase shares of our common stock in one or more offerings. Any decision to sell shares of our common stock below the then current net asset value per share of our common stock or securities to subscribe to, convert to, or

21

Table of Contents

purchase shares of our common stock would be subject to the determination by our Board of Directors that such issuance is in our and our stockholders' best interests.

        If we were to sell shares of our common stock below net asset value per share, such sales would result in an immediate dilution to the net asset value per share. This dilution would occur as a result of the sale of shares at a price below the then current net asset value per share of our common stock and a proportionately greater decrease in a stockholder's interest in our earnings and assets and voting interest in us than the increase in our assets resulting from such issuance. In addition, if we issue securities to subscribe to, convert to or purchase shares of common stock, the exercise or conversion of such securities would increase the number of outstanding shares of our common stock. Any such exercise would be dilutive on the voting power of existing stockholders, and could be dilutive with regard to dividends and our net asset value, and other economic aspects of the common stock.

        Because the number of shares of common stock that could be so issued and the timing of any issuance is not currently known, the actual dilutive effect cannot be predicted; however, the example below illustrates the effect of dilution to existing stockholders resulting from the sale of common stock at prices below the net asset value of such shares. Please see "Sales of Common Stock Below Net Asset Value" for a more complete discussion of the potentially dilutive impacts of an offering at a price less than net asset value, or NAV, per share.

        Illustration: Example of Dilutive Effect of the Issuance of Shares Below Net Asset Value.    Assume that Company XYZ has 1,000,000 total shares outstanding, $15,000,000 in total assets and $5,000,000 in total liabilities. The net asset value per share of the common stock of Company XYZ is $10.00. The following table illustrates the reduction to net asset value, or NAV, and the dilution experienced by Stockholder A following the sale of 40,000 shares of the common stock of Company XYZ at $9.50 per share, a price below its NAV per share.

Changes in laws or regulations governing our operations may adversely affect our business or cause us to alter our business strategy.

        We, the Funds, and our portfolio companies are subject to applicable local, state and federal laws and regulations, including, without limitation, federal immigration laws and regulations. New legislation may be enacted or new interpretations, rulings or regulations could be adopted, including those governing the types of investments we are permitted to make, any of which could harm us and our stockholders, potentially with retroactive effect. In addition, any change to the SBA's current debenture SBIC program could have a significant impact on our ability to obtain lower-cost leverage, through the Funds, and therefore, our ability to compete with other finance companies.

        Additionally, any changes to the laws and regulations governing our operations relating to permitted investments may cause us to alter our investment strategy in order to avail ourselves of new

22

Table of Contents

or different opportunities. Such changes could result in material differences to the strategies and plans set forth herein and may result in our investment focus shifting from the areas of expertise of our investment team to other types of investments in which our investment team may have less expertise or little or no experience. Thus, any such changes, if they occur, could have a material adverse effect on our results of operations and the value of your investment.

Terrorist attacks, acts of war or natural disasters may affect any market for our common stock, impact the businesses in which we invest and harm our business, operating results and financial condition.

        Terrorist acts, acts of war or natural disasters may disrupt our operations, as well as the operations of the businesses in which we invest. Such acts have created, and continue to create, economic and political uncertainties and have contributed to global economic instability. Future terrorist activities, military or security operations, or natural disasters could further weaken the domestic/global economies and create additional uncertainties, which may negatively impact the businesses in which we invest directly or indirectly and, in turn, could have a material adverse impact on our business, operating results and financial condition. Losses from terrorist attacks and natural disasters are generally uninsurable.

Risks Related to Our Investments

Our investments in portfolio companies involve higher levels of risk, and we could lose all or part of our investment.

        Investing in our portfolio companies involves a number of significant risks. Among other things, these companies:

•

may have limited financial resources and may be unable to meet their obligations under their debt instruments that we hold, which may be accompanied by a deterioration in the value of any collateral and a reduction in the likelihood of us realizing any guarantees from subsidiaries or affiliates of our portfolio companies that we may have obtained in connection with our investment, as well as a corresponding decrease in the value of the equity components of our investments;



•

may have shorter operating histories, narrower product lines, smaller market shares and/or significant customer concentrations than larger businesses, which tend to render them more vulnerable to competitors' actions and market conditions, as well as general economic downturns;



•

are more likely to depend on the management talents and efforts of a small group of persons; therefore, the death, disability, resignation, termination, or significant under-performance of one or more of these persons could have a material adverse impact on our portfolio company and, in turn, on us;



•

generally have less predictable operating results, may from time to time be parties to litigation, may be engaged in rapidly changing businesses with products subject to a substantial risk of obsolescence, and may require substantial additional capital to support their operations, finance expansion or maintain their competitive position; and



•

generally have less publicly available information about their businesses, operations and financial condition. We are required to rely on the ability of our management team and investment professionals to obtain adequate information to evaluate the potential returns from investing in these companies. If we are unable to uncover all material information about these companies, we may not make a fully informed investment decision, and may lose all or part of our investment.

23

Table of Contents

        In addition, in the course of providing significant managerial assistance to certain of our portfolio companies, certain of our officers and directors may serve as directors on the boards of such companies. To the extent that litigation arises out of our investments in these companies, our officers and directors may be named as defendants in such litigation, which could result in an expenditure of funds (through our indemnification of such officers and directors) and the diversion of management time and resources.

The lack of liquidity in our investments may adversely affect our business.

        We invest, and will continue to invest in companies whose securities are not publicly traded, and whose securities will be subject to legal and other restrictions on resale or will otherwise be less liquid than publicly traded securities. The illiquidity of these investments may make it difficult for us to sell these investments when desired. In addition, if we are required to liquidate all or a portion of our portfolio quickly, we may realize significantly less than the value at which we had previously recorded these investments. As a result, we do not expect to achieve liquidity in our investments in the near-term. Our investments are usually subject to contractual or legal restrictions on resale or are otherwise illiquid because there is usually no established trading market for such investments. The illiquidity of most of our investments may make it difficult for us to dispose of them at a favorable price, and, as a result, we may suffer losses.

We may not have the funds or ability to make additional investments in our portfolio companies.

        We may not have the funds or ability to make additional investments in our portfolio companies. After our initial investment in a portfolio company, we may be called upon from time to time to provide additional funds to such company or have the opportunity to increase our investment through the extension of additional loans, the exercise of a warrant to purchase common stock, or the funding of additional equity investments. There is no assurance that we will make, or will have sufficient funds to make, follow-on investments. Any decisions not to make a follow-on investment or any inability on our part to make such an investment may have a negative impact on a portfolio company in need of such an investment, may result in a missed opportunity for us to increase our participation in a successful operation or may reduce the expected yield on the investment.

Our portfolio companies may incur debt that ranks equally with, or senior to, our investments in such companies.

        We invest primarily in secured term debt as well as equity issued by LMM and middle market companies. Our portfolio companies may have, or may be permitted to incur, other debt that ranks equally with, or senior to, the debt in which we invest. By their terms, such debt instruments may entitle the holders to receive payment of interest or principal on or before the dates on which we are entitled to receive payments with respect to the debt instruments in which we invest. Also, in the event of insolvency, liquidation, dissolution, reorganization or bankruptcy of a portfolio company, holders of debt instruments ranking senior to our investment in that portfolio company would typically be entitled to receive payment in full before we receive any distribution. After repaying such senior creditors, such portfolio company may not have any remaining assets to use for repaying its obligation to us. In the case of debt ranking equally with debt instruments in which we invest, we would have to share on an equal basis any distributions with other creditors holding such debt in the event of an insolvency, liquidation, dissolution, reorganization or bankruptcy of the relevant portfolio company.

There may be circumstances where our debt investments could be subordinated to claims of other creditors or we could be subject to lender liability claims.

        Even though we may have structured certain of our investments as secured loans, if one of our portfolio companies were to go bankrupt, depending on the facts and circumstances, and based upon

24

Table of Contents

principles of equitable subordination as defined by existing case law, a bankruptcy court could subordinate all or a portion of our claim to that of other creditors and transfer any lien securing such subordinated claim to the bankruptcy estate. The principles of equitable subordination defined by case law have generally indicated that a claim may be subordinated only if its holder is guilty of misconduct or where the senior loan is re-characterized as an equity investment and the senior lender has actually provided significant managerial assistance to the bankrupt debtor. We may also be subject to lender liability claims for actions taken by us with respect to a borrower's business or instances where we exercise control over the borrower. It is possible that we could become subject to a lender's liability claim, including as a result of actions taken in rendering significant managerial assistance or actions to compel and collect payments from the borrower outside the ordinary course of business.

Second priority liens on collateral securing loans that we make to our portfolio companies may be subject to control by senior creditors with first priority liens. If there is a default, the value of the collateral may not be sufficient to repay in full both the first priority creditors and us.

        Certain loans that we make are secured by a second priority security interest in the same collateral pledged by a portfolio company to secure senior debt owed by the portfolio company to commercial banks or other traditional lenders. Often the senior lender has procured covenants from the portfolio company prohibiting the incurrence of additional secured debt without the senior lender's consent. Prior to and as a condition of permitting the portfolio company to borrow money from us secured by the same collateral pledged to the senior lender, the senior lender will require assurances that it will control the disposition of any collateral in the event of bankruptcy or other default. In many such cases, the senior lender will require us to enter into an "intercreditor agreement" prior to permitting the portfolio company to borrow from us. Typically the intercreditor agreements we are requested to execute expressly subordinate our debt instruments to those held by the senior lender and further provide that the senior lender shall control: (1) the commencement of foreclosure or other proceedings to liquidate and collect on the collateral; (2) the nature, timing and conduct of foreclosure or other collection proceedings; (3) the amendment of any collateral document; (4) the release of the security interests in respect of any collateral; and (5) the waiver of defaults under any security agreement. Because of the control we may cede to senior lenders under intercreditor agreements we may enter, we may be unable to realize the proceeds of any collateral securing some of our loans.

        Finally, the value of the collateral securing our debt investment will ultimately depend on market and economic conditions, the availability of buyers and other factors. Therefore, there can be no assurance that the proceeds, if any, from the sale or sales of all of the collateral would be sufficient to satisfy the loan obligations secured by our first or second priority liens. There is also a risk that such collateral securing our investments will decrease in value over time, will be difficult to sell in a timely manner, will be difficult to appraise and will fluctuate in value based upon the success of the portfolio company and market conditions. If such proceeds are not sufficient to repay amounts outstanding under the loan obligations secured by our second priority liens, then we, to the extent not repaid from the proceeds of the sale of the collateral, will only have an unsecured claim against the company's remaining assets, if any.

We are a non-diversified investment company within the meaning of the 1940 Act, and therefore we are not limited with respect to the proportion of our assets that may be invested in securities of a single issuer.

        We are classified as a non-diversified investment company within the meaning of the 1940 Act, which means that we are not limited by the 1940 Act with respect to the proportion of our assets that we may invest in securities of a single issuer. To the extent that we assume large positions in the securities of a small number of issuers, our net asset value may fluctuate to a greater extent than that of a diversified investment company as a result of changes in the financial condition or the market's assessment of the issuer. We may also be more susceptible to any single economic or regulatory

25

Table of Contents

occurrence than a diversified investment company. Beyond our RIC asset diversification requirements, we do not have fixed guidelines for diversification, and our investments could be concentrated in relatively few portfolio companies.

We generally will not control our portfolio companies.

        We do not, and do not expect to, control the decision making in many of our portfolio companies, even though we may have board representation or board observation rights, and our debt agreements may contain certain restrictive covenants. As a result, we are subject to the risk that a portfolio company in which we invest will make business decisions with which we disagree and the management of such company, as representatives of the holders of their common equity, will take risks or otherwise act in ways that do not serve our interests as debt investors. Due to the lack of liquidity for our investments in non-traded companies, we may not be able to dispose of our interests in our portfolio companies as readily as we would like or at an appropriate valuation. As a result, a portfolio company may make decisions that would decrease the value of our portfolio holdings.

Defaults by our portfolio companies will harm our operating results.

        A portfolio company's failure to satisfy financial or operating covenants imposed by us or other lenders could lead to non-payment of interest and other defaults and, potentially, termination of its loans and foreclosure on its secured assets, which could trigger cross-defaults under other agreements and jeopardize a portfolio company's ability to meet its obligations under the debt or equity securities that we hold. We may incur expenses to the extent necessary to seek recovery upon default or to negotiate new terms, which may include the waiver of certain financial covenants, with a defaulting portfolio company.

Any unrealized depreciation we experience on our loan portfolio may be an indication of future realized losses, which could reduce our income available for distribution.

        As a BDC, we are required to carry our investments at market value or, if no market value is ascertainable, at the fair value as determined in good faith by our Board of Directors. Decreases in the market values or fair values of our investments will be recorded as unrealized depreciation. Any unrealized depreciation in our loan portfolio could be an indication of a portfolio company's inability to meet its repayment obligations to us with respect to the affected loans. This could result in realized losses in the future and ultimately in reductions of our income available for distribution in future periods.

Prepayments of our debt investments by our portfolio companies could adversely impact our results of operations and reduce our return on equity.

        We are subject to the risk that the investments we make in our portfolio companies may be repaid prior to maturity. When this occurs, we will generally reinvest these proceeds in temporary investments, pending their future investment in new portfolio companies. These temporary investments will typically have substantially lower yields than the debt being prepaid and we could experience significant delays in reinvesting these amounts. Any future investment in a new portfolio company may also be at lower yields than the debt that was repaid. As a result, our results of operations could be materially adversely affected if one or more of our portfolio companies elect to prepay amounts owed to us. Additionally, prepayments could negatively impact our return on equity, which could result in a decline in the market price of our common stock.

26

Table of Contents

Changes in interest rates may affect our cost of capital and net investment income.

        Some of our debt investments will bear interest at variable rates and the interest income from these investments could be negatively affected by decreases in market interest rates. In addition, an increase in interest rates would make it more expensive for us to use debt to finance our investments. As a result, a significant increase in market interest rates could increase our cost of capital, which would reduce our net investment income. Also, an increase in interest rates available to investors could make an investment in our common stock less attractive if we are not able to increase our dividend rate, a situation which could reduce the value of our common stock. Conversely, a decrease in interest rates may have an adverse impact on our returns by requiring us to seek lower yields on our debt investments and by increasing the risk that our portfolio companies will prepay our debt investments, resulting in the need to redeploy capital at potentially lower rates. A decrease in market interest rates may also adversely impact our returns on idle funds, which would reduce our net investment income.

We may not realize gains from our equity investments.

        Certain investments that we have made in the past and may make in the future include warrants or other equity securities. Investments in equity securities involve a number of significant risks, including the risk of further dilution as a result of additional issuances, inability to access additional capital and failure to pay current distributions. Investments in preferred securities involve special risks, such as the risk of deferred distributions, credit risk, illiquidity and limited voting rights. In addition, we may from time to time make non-control, equity investments in portfolio companies. Our goal is ultimately to realize gains upon our disposition of such equity interests. However, the equity interests we receive may not appreciate in value and, in fact, may decline in value. Accordingly, we may not be able to realize gains from our equity interests, and any gains that we do realize on the disposition of any equity interests may not be sufficient to offset any other losses we experience. We also may be unable to realize any value if a portfolio company does not have a liquidity event, such as a sale of the business, recapitalization or public offering, which would allow us to sell the underlying equity interests. We often seek puts or similar rights to give us the right to sell our equity securities back to the portfolio company issuer; however, we may be unable to exercise these puts rights for the consideration provided in our investment documents if the issuer is in financial distress.

Our marketable securities and idle funds investments are subject to risks similar to our portfolio company investments.

        Marketable securities and idle funds investments can include, among other things, secured and unsecured debt investments, independently rated debt investments and diversified bond funds. Many of these investments in debt obligations are, or would be if rated, below investment grade quality. Indebtedness of below investment grade quality is regarded as having predominantly speculative characteristics with respect to the issuer's capacity to pay interest and repay principal, similar to our portfolio investments in our portfolio companies. See "—Our investments in portfolio companies involve higher levels of risk, and we could lose all or part of our investment." Many of these marketable securities and idle funds investments are purchased through over the counter or other markets and are therefore liquid at the time of purchase but may subsequently become illiquid due to events relating to the issuer of the securities, market events, economic conditions or investor perceptions. See "—The lack of liquidity in our investments may adversely affect our business" for a description of risks related to holding illiquid investments. In addition, domestic and foreign markets are complex and interrelated, so that events in one sector of the world markets or economy, or in one geographical region, can reverberate and have materially negative consequences for other market, economic or regional sectors in a manner that may not be foreseen and which may materially affect the market price of our marketable securities and idle funds investments. Other risks that our portfolio investments are subject to are also applicable to these marketable securities and idle funds investments.

27

Table of Contents

Risks Relating to Our Common Stock

Shares of closed-end investment companies, including BDCs, may trade at a discount to their net asset value.

        Shares of closed-end investment companies, including BDCs, may trade at a discount to net asset value. This characteristic of closed-end investment companies and BDCs 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 at, above or below net asset value. In addition, if our common stock trades below net asset value, we will generally not be able to issue additional common stock at the market price unless our stockholders approve such a sale and our Board of Directors makes certain determinations. See "—Risks Relating to Our Business and Structure—Stockholders may incur dilution if we sell shares of our common stock in one or more offerings at prices below the then current net asset value per share of our common stock or issue securities to subscribe to, convert to or purchase shares of our common stock" for a discussion of a proposal approved by our stockholders that permits us to issue shares of our common stock below net asset value.

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.

        Delays in investing the net proceeds raised in an offering or from exiting an investment or other capital may cause our performance to be worse than that of other fully invested BDCs or other lenders or investors pursuing comparable investment strategies. We cannot assure you that we will be able to identify any investments that meet our investment objective or that any investment that we make will produce a positive return. We may be unable to invest the net proceeds of any offering or from exiting an investment or other capital on acceptable terms within the time period that we anticipate or at all, which could harm our financial condition and operating results.

        We anticipate that, depending on market conditions and the amount of the capital, it may take us a substantial period of time to invest substantially all the capital in securities meeting our investment objective. During this period, we will invest the capital primarily in marketable securities and idle funds investments, which may produce returns that are significantly lower than the returns which we expect to achieve when our portfolio is fully invested in securities meeting our investment objective. As a result, any distributions that we pay during such period may be substantially lower than the distributions that we may be able to pay when our portfolio is fully invested in securities meeting our investment objective. In addition, until such time as the net proceeds of any offering or from exiting an investment or other capital are invested in securities meeting our investment objective, the market price for our common stock may decline. Thus, the initial return on your investment may be lower than when, if ever, our portfolio is fully invested in securities meeting our investment objective.

Investing in our common stock may involve an above average degree of risk.

        The investments we make in accordance with our investment objective may result in a higher amount of risk than alternative investment options and a higher risk of volatility or loss of principal. Our investments in portfolio companies involve higher levels of risk, and therefore, an investment in our shares may not be suitable for someone with lower risk tolerance.

The market price of our common stock may be volatile and fluctuate significantly.

        Fluctuations in the trading prices of our shares may adversely affect the liquidity of the trading market for our shares and, if we seek to raise capital through future equity financings, our ability to raise such equity capital. The market price and liquidity of the market for our common stock may be

28

Table of Contents

significantly affected by numerous factors, some of which are beyond our control and may not be directly related to our operating performance. These factors include:

•

significant volatility in the market price and trading volume of securities of BDCs or other companies in our sector, which are not necessarily related to the operating performance of these companies;



•

changes in regulatory policies, accounting pronouncements or tax guidelines, particularly with respect to RICs, BDCs or SBICs;



•

inability to obtain any exemptive relief that may be required by us in the future from the SEC;



•

loss of our BDC or RIC status or either of the Funds' status as an SBIC;



•

changes in our earnings or variations in our operating results;



•

changes in the value of our portfolio of investments;



•

any shortfall in our investment income or net investment income or any increase in losses from levels expected by investors or securities analysts;



•

loss of a major funding source;



•

fluctuations in interest rates;



•

the operating performance of companies comparable to us;



•

departure of our key personnel;



•

global or national credit market changes; and



•

general economic trends and other external factors.

Provisions of the Maryland General Corporation Law and our articles of incorporation and bylaws could deter takeover attempts and have an adverse impact on the price of our common stock.

        The Maryland General Corporation Law and our articles of incorporation and bylaws contain provisions that may have the effect of discouraging, delaying or making difficult a change in control of our company or the removal of our incumbent directors. The existence of these provisions, among others, may have a negative impact on the price of our common stock and may discourage third-party bids for ownership of our company. These provisions may prevent any premiums being offered to you for our common stock.

29

Table of Contents

CAUTIONARY STATEMENT CONCERNING FORWARD-LOOKING STATEMENTS

        Some of the statements in this prospectus and any accompanying prospectus supplement constitute forward-looking statements because they relate to future events or our future performance or financial condition. The forward-looking statements contained in this prospectus and any accompanying prospectus supplement 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 addition, words such as "anticipate," "believe," "expect" and "intend" indicate a forward-looking statement, although not all forward-looking statements include these words. The forward-looking statements contained in this prospectus and any accompanying prospectus supplement 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" and elsewhere in this prospectus and any accompanying prospectus supplement. 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.

        We have based the forward-looking statements included in this prospectus and will base the forward-looking statements included in any accompanying prospectus supplement on information available to us on the date of this prospectus and any accompanying prospectus supplement, as appropriate, and we assume no obligation to update any such forward-looking statements, except as required by law. Although we undertake no obligation to revise or update any forward-looking statements, whether as a result of new information, future events or otherwise, you are advised to consult any additional disclosures that we may make directly to you, including in the form of a prospectus supplement or post-effective amendment to the registration statement, or through reports that we in the future may file with the SEC, including annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K.

30

Table of Contents

USE OF PROCEEDS

        We intend to use the net proceeds from any offering to make investments in accordance with our investment objective and strategies described in this prospectus or any prospectus supplement, 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 Common Stock—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." The supplement to this prospectus relating to an offering will more fully identify the use of proceeds from such an offering.

PRICE RANGE OF COMMON STOCK AND DISTRIBUTIONS

        Our common stock is traded on the New York Stock Exchange ("NYSE") under the symbol "MAIN." Prior to October 14, 2010, our common stock was traded on the NASDAQ Global Select Market under the same symbol "MAIN." Our common stock began trading on the NASDAQ Global Select Market on October 5, 2007. Prior to that date, there was no established public trading market for our common stock.

        The following table sets forth, for each fiscal quarter since our common stock began trading, the range of high and low closing prices of our common stock as reported on the NYSE and on the

31

Table of Contents

NASDAQ Global Select Market, as applicable, and the sales price as a percentage of the net asset value per share of our common stock ("NAV").

        On May 22, 2012, the last sale price of our common stock on the NYSE was $23.38 per share, and there were approximately 205 holders of record of the common stock which did not include shareholders for whom shares are held in "nominee" or "street name."

        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.

32

Table of Contents

        From our IPO through the third quarter of 2008, we paid quarterly dividends, but in the fourth quarter of 2008 we began paying, and we intend to continue paying, monthly dividends to our stockholders. Our monthly dividends, if any, will be determined by our Board of Directors on a quarterly basis.

        The following table summarizes our dividends declared to date:

33

Table of Contents

        To obtain and maintain RIC tax treatment, we must, among other things, distribute at least 90% of our net ordinary income and realized net short-term capital gains in excess of realized net long-term capital losses, if any. We will be subject to a 4% nondeductible federal excise tax on certain undistributed taxable income unless we distribute in a timely manner an amount at least equal to the sum of (1) 98% of our net ordinary income for each calendar year, (2) 98.2% of our capital gain net income for the one-year period ending December 31 in that calendar year and (3) any income recognized, but not distributed, in preceding years (the "Excise Tax Avoidance Requirement"). Dividends declared and paid by us in a year will generally differ from taxable income for that year, as such dividends may include the distribution of current year taxable income, less amounts carried over into the following year, and the distribution of prior year taxable income carried over into and distributed in the current year. For amounts we carry over into the following year, we will be required to pay a 4% excise tax for the excess over 98% of our annual taxable income in excess of distributions for the year. We may retain for investment some or all of our net capital gains (i.e., realized net long-term capital gains in excess of realized net short-term capital losses) and treat such amounts as deemed distributions to our stockholders. If we do this, our stockholders will be treated as if they had received actual distributions of the capital gains we retained and then reinvested the net after-tax proceeds in our common stock. In general, our stockholders also would be eligible to claim a tax credit (or, in certain circumstances, a tax refund) equal to their allocable shares of the tax we paid on the capital gains deemed distributed to them. We can offer no assurance that we will achieve results that will permit the payment of any cash distributions and, if we issue senior securities, we may be prohibited from making distributions if doing so causes us to fail to maintain the asset coverage ratios stipulated by the 1940 Act or if distributions are limited by the terms of any of our borrowings.

        We may distribute taxable dividends that are payable in cash or shares of our common stock at the election of each stockholder. Under certain applicable provisions of the Code and the Treasury regulations, distributions payable in cash or in shares of stock at the election of stockholders are treated as taxable dividends. The Internal Revenue Service has issued private rulings indicating that this

34

Table of Contents

rule will apply even where the total amount of cash that may be distributed is limited to no more than 20% of the total distribution. Under these rulings, if too many stockholders elect to receive their distributions in cash, each such stockholder would receive a pro rata share of the total cash to be distributed and would receive the remainder of their distribution in shares of stock. If we decide to make any distributions consistent with these rulings that are payable in part in our stock, taxable stockholders receiving such dividends will be required to include the full amount of the dividend (whether received in cash, our stock, or a combination thereof) as ordinary income (or as long-term capital gain to the extent such distribution is properly reported as a capital gain dividend) to the extent of our current and accumulated earnings and profits for United States federal income tax purposes. As a result, a U.S. stockholder may be required to pay tax with respect to such dividends in excess of any cash received. If a U.S. stockholder sells the stock it receives in order to pay this tax, the sales proceeds may be less than the amount included in income with respect to the dividend, depending on the market price of our stock at the time of the sale. Furthermore, with respect to non-U.S. stockholders, we may be required to withhold U.S. tax with respect to such dividends, including in respect of all or a portion of such dividend that is payable in stock. In addition, if a significant number of our stockholders determine to sell shares of our stock in order to pay taxes owed on dividends, it may put downward pressure on the trading price of our stock.

        Where the IRS revenue procedure is not currently applicable, the IRS has also issued private letter rulings on cash and stock dividends paid by RICs and real estate investment trusts using a 20% cash standard (and, more recently, the 10% cash standard of the above referenced IRS revenue procedure) if certain requirements are satisfied.

35

Table of Contents

SELECTED FINANCIAL DATA

        The selected financial and other data below reflects the consolidated operations of Main Street and its subsidiaries for the years ended December 31, 2007, 2008, 2009, 2010, and 2011 and for the three months ended March 31, 2011 and 2012. The selected financial data at December 31, 2007, 2008, 2009, 2010, and 2011 and for the years ended December 31, 2007, 2008, 2009, 2010, and 2011, have been derived from consolidated financial statements that have been audited by Grant Thornton LLP, an independent registered public accounting firm. The selected financial data at March 31, 2012, and for the three months ended March 31, 2011 and 2012, have been derived from unaudited financial data but, in the opinion of management, reflect all adjustments (consisting only of normal recurring adjustments) that are necessary to present fairly the financial condition and operating results for such interim periods. Interim results as of and for the three months ended March 31, 2012 are not necessarily indicative of the results that may be expected for the year ending December 31, 2012. You should read this selected financial data in conjunction with our "Management's Discussion and Analysis of Financial Condition and Results of Operations," "Senior Securities" and the financial statements and related notes included in this prospectus.