Sullivan began to fight professionally in 1878 after briefly studying at Boston College. On Feb. 7, 1882, at Mississippi City, Miss., he knocked out Paddy Ryan in the ninth round of a bareknuckle fight for the heavyweight championship. His 75-round knockout of Jake Kilrain at Richburg, Miss., July 8, 1889, was the last heavyweight title bout under London Prize Ring (London Prize Ring rules) (bareknuckle) rules. In his only championship defense under the Queensberry rules (Marquess of Queensberry rules), Sullivan was knocked out by James J. Corbett (Corbett, James J.) in 21 rounds in New Orleans, La., Sept. 7, 1892.

Although Sullivan is generally considered to have been world heavyweight champion, some boxing historians regard him as a U.S. champion only. His only international match of consequence was with the English pugilist Charley Mitchell at Chantilly, Oise, Fr., March 10, 1888; it ended as a draw after 39 rounds. In addition, Sullivan declined to fight the great Australian black heavyweight Peter Jackson (Jackson, Peter). From 1878 to 1905 Sullivan had 35 bouts, winning 31, of which 16 were by knockouts.

Sullivan's blustering personality and fabled strength gained him a great following. As champion he earned more than $1 million but squandered it. A hard drinker during his ring career, he later became an advocate of prohibition and delivered many lectures on the subject.