Exhibit 99.1

NEWS RELEASE

Marsh & McLennan Companies Announces Pricing of $600 Million Senior Notes Offering

NEW YORK, September 9, 2015 — Marsh & McLennan Companies, Inc. (the “Company”) announced today that it has priced $600 million of 3.750% senior notes due 2026 (the “Notes”). The Company intends to use the net proceeds for general corporate purposes. The closing of the Notes offering is expected to occur on September 14, 2015, subject to certain customary conditions. Merrill Lynch, Pierce, Fenner & Smith Incorporated, Goldman, Sachs & Co., J.P. Morgan Securities LLC and Morgan Stanley & Co. LLC are acting as joint book-running managers for the Notes offering. ANZ Securities, Inc., GC Securities, a division of MMC Securities Corp., Scotia Capital (USA) Inc. and Wells Fargo Securities, LLC are acting as co-managers for the Notes offering.

An effective shelf registration statement related to the Notes has previously been filed with the Securities and Exchange Commission (the “SEC”). The offering and sale of the Notes are being made by means of a prospectus supplement and an accompanying base prospectus related to the offering. Before you invest, you should read the prospectus supplement and the base prospectus for more complete information about the issuer and this offering. You may obtain these documents for free by visiting EDGAR or the SEC website at www.sec.gov; alternatively, copies may be obtained from: (i) Merrill Lynch, Pierce, Fenner & Smith Incorporated, 222 Broadway, New York, NY 10038, Attn: Prospectus Department, telephone: 1-800-294-1322 or by email at dg.prospectus_requests@baml.com, (ii) Goldman, Sachs & Co., Prospectus Department, 200 West Street, New York, New York, 10282, telephone: 1-866-471-2526, facsimile: (212) 902-9316 or by email at prospectus-ny@ny.email.gs.com,

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(iii) J.P. Morgan Securities LLC, 383 Madison Avenue, New York, New York 10179, Attn: Investment Grade Syndicate Desk — 3^rd Floor, telephone: 1-212-834-4533 and (iv) Morgan Stanley & Co. LLC, 180 Varick Street, New York, New York 10014, Attn: Prospectus Department, telephone: 1-866-718-1649 or by email at prospectus@morganstanley.com.

This press release does not constitute an offer to sell or a solicitation of an offer to buy any securities, nor does it constitute an offer, solicitation or sale in any jurisdiction in which such offer, solicitation or sale is unlawful.

About Marsh & McLennan Companies

MARSH & McLENNAN COMPANIES (NYSE: MMC) is a global professional services firm offering clients advice and solutions in the areas of risk, strategy and people. Marsh is a leader in insurance broking and risk management; Guy Carpenter is a leader in providing risk and reinsurance intermediary services; Mercer is a leader in talent, health, retirement and investment consulting; and Oliver Wyman is a leader in management consulting. With annual revenue of $13 billion and 57,000 colleagues worldwide, Marsh & McLennan Companies provides analysis, advice and transactional capabilities to clients in more than 130 countries. The Company is committed to being a responsible corporate citizen and making a positive impact in the communities in which it operates.

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