He studied fine art in London and Paris from 1898 to 1900, after which he returned to the United States, where he produced countless illustrations for books, magazine covers, political and humorous cartoons, advertising, and spot drawings.

Among his creations was a comic strip that appeared regularly in Judge from 1903 until 1907, about a tramp character titled Nervy Nat.

[4][5] In 1915, he accepted commissions from Calkins and Holden to create advertisements for Edison Photo and Adler Rochester Overcoats but only on the condition that his name would not be associated with the campaign.

[7] Flagg had first created the image for the July 6, 1916, cover of Frank Leslie's Illustrated Newspaper with the headline "What Are You Doing for Preparedness?

Flagg used his own face for that of Uncle Sam (adding age and the white goatee), he said later, simply to avoid the trouble of arranging for a model.

Flagg had a neighbor, Walter Botts, pose as a model for the strong shoulders, and thrusting forefinger of the piece.

The artists-only event was held in the 1845-built Hotel Brevoort in Greenwich Village, a neighborhood where many artists and cartoonists lived at the time.