At his peak, Flagg was reported to have been the highest-paid magazine illustrator in America. He worked for the "Saturday Evening Post" and "Collier's" which were two of the most popular U.S. journals. In 1946, Flagg published his autobiography, "Roses and Buckshot". Apart from his work as an illustrator, Flagg painted portraits which reveal the influence of John Singer Sargent. Flagg's sitters included Mark Twain and Ethel Barrymore; his portrait of Jack Dempsey now hangs in the Great Hall of the National Portrait Gallery. In 1948, he appeared in a Pabst Blue Ribbon magazine ad which featured the illustrator working at an easel in his New York studio with a young lady standing at his side and a tray with an open bottle of Pabst and two filled glasses sat before them.