The association of the government of the United States of America with "Uncle Sam" dates to the War of 1812. During the conflict, Samuel Wilson of New York was supplying beef to the army with packaging stamped "U.S." Soldiers joked that this meant "Uncle Sam's," a reference to Samuel Wilson, which newspapers reported.

The public subsequently adopted the idea of referring to the "general government" – as the federal government was known at the time – as "Uncle Sam." The character did not assume the image now popularly recognized until after the Civil War when Thomas Nast (1840-1902), a newspaper cartoonist, created the image of an old man with a white goatee and wearing a suit with stars and stripes; coincidentally Nast also invented the donkey and elephant symbols for the American political parties and he created much of the modern image of Santa Claus during the same period. Nast apparently drew upon a Know-Nothing party cartoon of 1855 depicting Brother Jonathan as well as caricatures of Uncle Sam and Brother Jonathan that had appeared in the British humor magazine, Punch.

This Wiki page has an illustration of Brother Jonathan. It also shows this character with Mrs. Britannia and Miss Canada, both nations anthropomorphized in a cartoon in 1886. Political cartoons of the nineteenth century relied on these sorts of characters to drive home points about international affairs. Perhaps someone else can answer your question about how early this phenomenon can be found.

See: Winifred Morgan, An American Icon: Brother Jonathan and American Identity (Newark: University of Delaware Press, 1988).