Mountfort Coolidge was born in Brooklyn, New York in 1888 and was a pupil of Robert Henri's. In the early teens he travelled with his family to Ogunquit, Maine, then already a thriving artist's community, and became a student of Hamilton Faster Field's. He continued to summer in Ogunquit for over 40 years, and was often associated with the flamboyant society painter Channing Hare, with whom he operated a small antiques business.

The two men shared a home on Pine Hill North. OAA (Ogunquit Art Associatio) members Channing Hare and Mountfort Coolidge, although respected and successful painters in their own right, were identified more with the summer society life than it's art colony by many artists and local residents. Close friends, fellow artists and even business partners, the two men were synonymous with glamour, flamboyance, and chic.

Despite Hare's seeming preoccupation with the gathering of interesting people for elaborate cocktail parties, he was a brilliant society portrait painter. His clients Included such noted Palm Beach and New York socialites as Phyllis Rhinelander and Alexander Woolcott, comedienne Beatrice Lily, actress Florence Nash, and authors Booth Tarkington and Kenneth Roberts.

More subdued than his partner, Coolidge spent most of his time at work, either painting for shows at such prestigious New York midtown galleries as the Kraushaar and Kleeman's or in the antique shop.

Mountfort Coolidge painted tirelessly for several decades. He became a member of the Ogunquit Art Association, exhibiting locally. He also had several successful exhibitions in Paris, New York, (Kraushaar Gallery, Boston (Margaret Brown Gallery), Palm Beach (Society of the Four Arts), and elsewhere. He died in Ogunquit, Maine, in 1954.