Marsh & McLennan Companies, Inc. is a global professional services firm, headquartered in New York City with businesses in insurance brokerage, risk management, reinsurance services, talent management, investment advisory, and management consulting.

Its four main operating companies are Marsh, Mercer, Oliver Wyman Group, and Guy Carpenter.

Marsh & McLennan Companies ranked #210 on the 2017 Fortune 500 ranking, the company's 23rd year on the annual Fortune list, and #458 on the 2017 Forbes Global 2000 List.

Marsh & McLennan's 2016 revenue of $13.2 billion kept it #1 on Business Insurance's ranking of the world's largest insurance brokers.

Burroughs, Marsh & McLennan was formed by Henry W. Marsh and Donald R. McLennan in Chicago in 1905. It was renamed as Marsh & McLennan in 1906.

The reinsurance firm Guy Carpenter & Company was acquired in 1923, a year after it was founded by Guy Carpenter. In 1959, it acquired the human resources consulting firm Mercer. In 1997, it bought Johnson & Higgins. Shortly after, it bought Sedgwick.

On October 11, 2001, Marsh established a crisis consulting practice specializing in terrorism, with Ambassador L. Paul Bremer as Chairman. Marsh also announced a partnership with Control Risks Group to provide political risk assessment.

In 2003, it purchased the consultancy business Oliver, Wyman & Company and merged these with some of the consultancy businesses from previous acquisitions, particularly the Mercer business under the brand name of Oliver Wyman.

On July 8, 2004, Marsh completed the acquisition of Kroll Inc. Jeffrey W. Greenberg called it an important strategic step. The company had employed terrorism expert John O'Neill, formerly of the Federal Bureau of Investigation. On October 14, 2004, New York State Attorney General Eliot Spitzer announced the initiation of a civil action against Marsh, alleging impropriety in the steering of clients to insurers with whom the company maintained payoff agreements, and for soliciting rigged bids for insurance contracts from the insurers. The Attorney General announced that two AIG executives pleaded guilty to criminal charges in connection with this illegal course of conduct and stated, "There is simply no responsible argument for a system that rigs bids, stifles competition and cheats customers." CEO Jeffrey W. Greenberg resigned several weeks later. The suit was ultimately settled out of court.

On January 31, 2005, Marsh & Mclennan agreed on an $850-million settlement for its bid-rigging practices.

In August 2007, Marsh completed the sale of its Putnam Investments division to Great-West Lifeco Inc., a financial services holding company controlled by Canada-based Power Financial Corp. after it settled charges of market timing. On September 14, 2007, Brian M. Storms, the CEO of Marsh's insurance brokerage unit, resigned. As Michael G. Cherkasky explained his departure, "we now need a different set of leadership and operational skills".

In July 2017, Marsh & McLennan Cos. Inc. was ranked first in Business Insurance's world's largest brokers list.

At the time of the September 11 attacks in the United States in 2001, the corporation held offices on eight floors of the North Tower of the World Trade Center, from 93 to 100. When American Airlines Flight 11 crashed into the building, their offices spanned the entire impact zone, from floors 93 to 99. Everybody present in their offices on that day of the attack died as all stairwells at impact zone were destroyed/blocked by the plane crash, and the firm lost 295 employees and 63 contractors. (Cantor Fitzgerald is the only company to have lost more in the attack.)

Marsh & McLennan Companies is composed of two primary business segments: Risk and Insurance Services, and Consulting.