Following the retirement of Gwyn Tompkins, American Flag was sent back to the track at age four under trainer George Conway.

In the 1926 Suburban Handicap American Flag ran second to stablemate Crusader, who was another son of Man o' War.

Retired to his owner's Faraway Farm in Lexington, Kentucky, as a stallion, American Flag met with modest success.

His most notable offspring was the colt Gusto who won the American Derby, Arlington Classic Stakes and the Jockey Club Gold Cup, and through his mating to the mare Nellie Morse, the filly Nellie Flag who was the 1934 U.S. Champion Two-Year-Old Filly and who became a significant broodmare for Calumet Farm.

In 1942, during World War II, owner Samuel Riddle gave American Flag to the U.S. Army Remount Service.