Hart, nicknamed "the Louisville Plumber" because of his former trade, gained considerable prominence after a 1905 win over future champion Jack Johnson.

That year, the heavyweight title was left vacant as a result of the retirement of champion James J. Jeffries and Hart's record earned him a chance to fight for the championship against top-ranked Jack Root (1876–1963), a much more experienced boxer, who had already beaten Hart in November, 1902.

Jeffries, the retiring champ, refereed the championship fight on July 3, 1905, in Reno, Nevada.

After one successful exhibition match, Hart lost his championship to Canadian Tommy Burns on February 23, 1906, in Los Angeles.

Hart died the day after his 55th birthday of an enlarged liver and high blood pressure.