Uncle Sam is a real guy and his poster is a self-portrait
In 1917, artist James Montgomery Flagg created his most famous work, a recruiting poster for the U.S. Army featuring a white-haired, white-whiskered man in an old-timey (even by the standards of the day) top hat, coat, and tie in bold red, white, and blue colors. Inspired by similar recruiting posters in Europe at the time, the poster was adapted to appeal to everyday Americans, along with their sense of individuality and patriotism. It has become one of the most enduring symbols of the United States military.
And it's basically a portrait of Flagg himself.
And that's how you achieve immortality.
Flagg's stock in trade was creating cartoons, illustrations, and drawings for publications of all sorts. He worked for advertising firms, newspapers, book publishers, and other creators who required illustrations such as Flagg's. He was commissioned to create the cover for Frank Leslie's Illustrated Newspaper in 1916. It was a weekly publication that pioneered the use of early photography to illustrate American life during its 70-plus year run, and he used himself as a model. Hearkening back to the early days of the magazine, he chose to depict himself as an older gentleman in an outdated, if colorful outfit.
The headline of that week's issue was "What Are You Doing For Preparedness?" He decided to make the poster a reference to a then-famous recruiting poster for the British Army, one that depicted the famous Field Marshal Lord Herbert Kitchener, pointing at the viewer and telling them they're wanted in the British Army, using the likeness of Uncle Sam in the place of Kitchener.
As for the origin of Uncle Sam, the true origin is disputed. A resolution from Congress in 1961 declared that an Upstate New York meat inspector named Sam Wilson was the original Uncle Sam. Wilson was a Continental Army veteran from Troy, New York, who provided rations to the Army during the War of 1812. It's not known whether Wilson's appearance was the inspiration for the rest of Uncle Sam's appearance, but Flagg's depiction of himself as Uncle Sam certainly stood the test of time.
Flagg's painting was reused again as a recruiting tool during World War II, and the notoriety from his work earned him a place as one of the top illustrators of the day, working for the best magazines and newspapers who could afford work like his. He even went on to paint portraits of famous Americans that would end up in the National Portrait Gallery, such as Mark Twain and boxer Jack Dempsey. Flagg died in 1960, the year before Congress decided to honor Sam Wilson as the true "Uncle Sam."