From "Legends, Lies & Cherished Myths of American History" by Richard Shenkman:

Uncle Sam was a real person, too, by the name of Sam Wilson and, as fate would have it, one of Johnny Appleseed's boyhood friends. Wilson didn't wear striped pants and didn't have a long white beard. But he did wear a top hat and in his own day became a symbol of the United States government. The identification with the government came during the War of 1812, when Wilson began supplying meat to troops stationed around Troy, New York. Meat sent to the soldiers was stamped "U.S." for United States. But when a government inspector came along to check on the meat, he was told by an imaginative worker in Wilson's store that the initials stood for "Uncle Sam," Wilson's nickname. Soon all federal supplies were said to belong to "Uncle Sam."

For many years scholars discounted claims that Uncle Sam had been named in honor of Sam Wilson, but in 1961 a scholar discovered an 1830 newspaper which gave credence to the story. The newspaper quoted a soldier from the War of 1812 who claimed to have been at Wilson's store the day the inspector was told about the initials. In the early 1960's the U.S. Congress officially proclaimed Wilson the original Uncle Sam.