Jacob Finkelstein began boxing as a youth of 14 in Chicago. He was trained by a former fighter, Marty Fields, and he shortly adopted Fields’ name as his own. Jackie Fields was a fine amateur, winning 51 of 54 bouts in addition to his Olympic glory, but his Olympic featherweight title was only the prelude to a great career. He turned professional shortly after returning from Paris and went on to twice become world welterweight champion. He first won the National Boxing Association version of the title on 25 March 1929, when he beat Young Jack Thompson in 10 rounds. On 25 July 1929 he laid claim to the undisputed title by winning on a second-round foul from Joe Dundee. Fields later lost his title to Thompson in 1930 but he regained it in 1932, by defeating Lou Brouillard in 10 rounds. After losing the title in 1933 to Young Corbett, Fields fought only one more time and then retired.