U.S. racehorse trainer who, during his 78-year career, trained the winners of 2,275 races, bringing in purses totalling more than $13,000,000. He trained more than 250 winners of stakes events, including two winners of the U.S. Triple Crown (the Kentucky Derby, the Preakness, and the Belmont Stakes): Gallant Fox (1930) and Gallant Fox's colt Omaha (1935). Among the other outstanding horses he trained—victors in most of the leading stakes races in North America—were Fairene, Fenelon, Fighting Fox, Vagrancy, Johnstown, Nashua, Bold Ruler, and Granville.

Fitzsimmons began his career in 1885 as a stable boy at Brannon Brothers stable. After a moderately successful career as jockey (1889–94), which he had to abandon because of increasing weight, he became a trainer and in 1900 produced his first winner, Agnes D. His career of nearly 70 years included lengthy associations with the Belsir Stud and Wheatley stables. He retired in 1963.