MMR's team is being merged into the Ipsos global media research team, to give critical mass to the specialist MediaCT practice, which covers North America.
MMR is based in New York City, and was founded in 1958. It has revenues of over $10m (£5.1m) and 20 full-time employees. It publishes the Mendelsohn Affluent Survey, which looks at the behaviour of wealthy households. Its clients include Conde Nast, Time Warner, Dow Jones and Hachette.
Following the merger, it will become known as Ipsos Mendelsohn, with current president and CEO Walter McCullough remaining in charge.
Jim Smith, chairman and CEO of Ipsos in North America, said: "With technology advancing at a breakneck speed, the media business in North America is evolving at an astonishing pace.
"Folding Monroe Mendelsohn's knowledge and experience into the Ipsos stable is a major advantage for our media clients and potential business partners."