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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-K

(Mark One)

For the fiscal year ended December 31, 2025

OR

Commission File No. 1-5998

Marsh & McLennan Companies, Inc.

(Exact name of registrant as specified in its charter)

1166 Avenue of the Americas

New York, New York 10036-2774

(Address of principal executive offices; Zip Code)

(212) 345-5000

Registrant’s telephone number, including area code

Securities registered pursuant to Section 12(b) of the Act:

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act. Yes ý No ¨

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act. Yes ¨ No ý

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days. Yes ý No ¨

Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the registrant was required to submit such files). Yes ý No ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer or a smaller reporting Company. See the definitions of "large accelerated filer," "accelerated filer" and "smaller reporting Company" in Rule 12b-2 of the Exchange Act. (Check one):

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act. ☐

Indicate by check mark whether the registrant is a shell Company (as defined in Rule 12b-2 of the Exchange Act). Yes ☐ No ý

Indicate by check mark whether the registrant has filed a report on and attestation to its management’s assessment of the effectiveness of its internal control over financial reporting under Section 404(b) of the Sarbanes-Oxley Act (15 U.S.C. 7262(b)) by the registered public accounting firm that prepared or issued its audit report. Yes ☒ No ¨

If securities are registered pursuant to Section 12(b) of the Act, indicate by check mark whether the financial statements of the registrant included in the filing reflect the correction of an error to previously issued financial statements. ¨

Indicate by check mark whether any of those error corrections are restatements that required a recovery analysis of incentive- based compensation received by any of the registrant’s executive officers during the relevant recovery period pursuant to §240.10D-1(b). ¨

As of June 30, 2025, the aggregate market value of the registrant's common stock held by non-affiliates of the registrant was approximately $107,422,546,097 computed by reference to the closing price of such stock as reported on the New York Stock Exchange on June 30, 2025.

As of February 5, 2026, there were outstanding 484,124,253 shares of common stock, par value $1.00 per share, of the registrant.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of Marsh & McLennan Companies, Inc.’s Notice of Annual Meeting and Proxy Statement for the 2026 Annual Meeting of Stockholders (the "2026 Proxy Statement") are incorporated by reference in Part III of this Form 10-K.

INFORMATION CONCERNING FORWARD-LOOKING STATEMENTS

This Annual Report on Form 10-K contains "forward-looking statements," as defined in the Private Securities Litigation Reform Act of 1995. These statements, which express management's current views concerning future events or results, use words like "anticipate," "assume," "believe," "continue," "estimate," "expect," "intend," "plan," "project" and similar terms, and future or conditional tense verbs like "could," "may," "might," "should," "will" and "would".

Forward-looking statements are subject to inherent risks and uncertainties that could cause actual results to differ materially from those expressed or implied in our forward-looking statements. Factors that could materially affect our future results include, among other things:

• the impact of geopolitical or macroeconomic conditions on us, our clients and the countries and industries in which we operate, including from multiple major wars and global conflicts, social unrest, tariffs or changes in trade policy, slower GDP growth or recession, fluctuations in foreign exchange rates, lower interest rates, capital markets volatility, inflation and changes in insurance premium rates;

• the impact from lawsuits or investigations arising from errors and omissions, breaches of fiduciary duty or other claims against us in our capacity as a broker or investment advisor, including claims related to our investment business’ ability to execute timely trades;

• the increasing prevalence of ransomware, supply chain and other forms of cyberattacks, and their potential to disrupt our operations or the operations of our third party vendors, and result in the disclosure of confidential client or company information;

• the financial and operational impact of complying with laws and regulations, including domestic and international sanctions regimes, anti-corruption laws such as the U.S. Foreign Corrupt Practices Act, U.K. Anti Bribery Act and cybersecurity, data privacy and artificial intelligence regulations;

• our ability to attract, retain and develop industry leading talent;

• our ability to compete effectively and adapt to competitive pressures in each of our businesses, including from disintermediation as well as technological change, digital disruption and other types of innovation such as artificial intelligence;

• our ability to manage potential conflicts of interest, including where our services to a client conflict, or are perceived to conflict, with the interests of another client or our own interests;

• our ability to fully realize the opportunities and efficiencies from the Thrive program, which focuses on our brand strategy, delivering greater value to clients, accelerating growth and improving efficiency;

• the regulatory, contractual and reputational risks that arise based on insurance placement activities and insurer revenue streams; and

• the impact of changes in tax laws, guidance and interpretations, such as the implementation of the Organization for Economic Cooperation and Development international tax framework, or the increasing number of challenges by tax authorities in the current global tax environment.

The factors identified above are not exhaustive. Further information concerning Marsh and its businesses, including information about factors that could materially affect our results of operations and financial condition, is contained in the Company's filings with the Securities and Exchange Commission, including the "Risk Factors" section in Part I, Item 1A of this report and the "Management’s Discussion and Analysis of Financial Condition and Results of Operations" section in Part II, Item 7 of this report. Marsh and its subsidiaries operate in a dynamic business environment in which new risks emerge frequently. Accordingly, we caution readers not to place undue reliance on any forward-looking statements, which are based only on information currently available to us and speak only as of the dates on which they are made. The Company undertakes no obligation to update or revise any forward-looking statement to reflect events or circumstances arising after the date on which it is made.

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TABLE OF CONTENTS

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PART I