Flagg was born in Pelham Manor, New York. He was enthusiastic about drawing from a young age, and had illustrations accepted by national magazines by the age of 12 years. By 14 he was a contributing artist for Life Magazine, and the following year was on the staff of Judge Magazine. He studied fine art in London and Paris in his early 20s, then retured to the United States, where he produced illustrations for books, magazine covers, political and humorous cartoons, advertising, and spot drawings prolifically.
His most famous poster was created in 1917 to encourage recruitment in the United States Army during World War I. It showed Uncle Sam pointing at the viewer (inspired by a British recruitment poster showing Lord Horatio Kitchener in a similar pose) with the caption "I Want YOU for the U. S. Army". Over 4 million copies of the poster were printed during World War I, and it was revived for World War II. Flagg used his own face for that of Uncle Sam (adding age and the white beard), he said later simply to avoid the trouble of arranging for a model.
In 1946 Flagg published his autobiography, Roses and Buckshot.
James Montgomery Flagg died in New York City