He's even made it into a few other comic book companys...like Marvel's Captain America and Fawcett's Captain Marvel...
A Quality Uncle Sam
Uncle Sam's origin appeared in National Comics #5 (November, 1940), with story by Will Eisner, and cover and interior art by Lou Fine (this issue also introduced Quality's speed hero, Quicksilver....).
Uncle Sam stayed to fight for America from National Comics #1-45 (that last issue in December, 1944, and Uncle Sam was on the covers up to #41 before being replaced by the Barker...).
During this time, Uncle Sam proved popular enough to even get his own book as well!
Uncle Sam Breaks Loose
Sam's last Quality appearance was in National Comics #45 (December, 1944)...but that wasn't the last appearance of Uncle Sam. DC Comics had purchased the Quality characters, and kept the Blackhawks, G.I. Combat, Heart Throbs and Robin Hood Tales titles going under the DC banner (or is that checker?)...and even revived Plastic Man in the 1960s...but as for the rest...their freedom came in the 1970s...
Freedom for the Quality Heroes
While the JLA stories are reprinted in Crisis On Multiple Earths 3 under an Alex Ross cover, the Freedom Fighters tales have yet to be collected (and are wanted....)
More Uncle Sam History
TwoMorrows, by Mike Kooiman and Jim Amash with a cover by Dick Giordano!