American world lightweight (135-lb 【61.2-kg】) boxing champion from May 28, 1917, when he knocked out Freddy Welsh in nine rounds in New York City, until Jan. 15, 1925, when he retired. He is regarded as one of the cleverest defensive boxers in the history of professional boxing.

A professional fighter from 1911 to 1942, he had 210 bouts, winning 89 (45 by knockouts), with 115 no-decision bouts. He was noted for distracting his opponents by talking to them. Leonard retired after successfully defending the lightweight title seven times and losing on a foul in an attempt to win the welterweight (147-lb) championship from Jack Britton (June 26, 1922). In 1931–32, after several years of inactivity, he had numerous fights in the welterweight division, but he retired again after being knocked out by Jimmy McLarnin on Oct. 7, 1932. He died while refereeing a bout in the St. Nicholas Arena, New York.