Trained by Gwyn Tompkins, as a two year old American Flag's most important performance was a win in the Manor Stakes at Laurel Park Racecourse in Laurel, Maryland. Undefeated at age three, he had important wins in the Withers and Dwyer Stakes plus he captured what became known as the third leg of the U.S. Triple Crown series, the Belmont Stakes. His 1925 performances earned him Champion Three-Year-Old colt honors.
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. He 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.