McLarnin defeated already at the beginning of his career at featherweight Pancho Villa. In 1928 he suffered against Ray Miller his only defeat early, but Miller was able to defeat on points in the rematch. 1929 and 1930 he won against Sammy Mandell on points. Billy Petr Olle, who had defeated him in 1930, he hit 1931 twice. The Lightweight legend Benny Leonard, he defeated 1932 prematurely.
1933 McLarnin won the welterweight world title with a first-round KO against Young Corbett III. In October 1933, he participated in the US- American citizenship. 1934 and 1935 he played a series of three battles against Barney Ross. He lost the first meeting, won the second fight, but was defeated in the third battle and lost his title thus definitively.
Then he lost to Tony Canzoneri, but won the rematch. After a points victory over former world champion in the Lightweight Lou Ambers he finished his career with 29 years.
McLarnin was despite its low for today's standards KO ratio as a boxer with a good punch. The Ring Magazine ranks addition to the aforementioned adversaries Kid Kaplan (1927, KO8 ), Sid Terris (1928, KO1 ), Joe Glick (1929, W points and KO2 ), Sammy Baker (1929, KO1 ), Ruby Goldstein (1929, KO2 ) and Al Singer (1930, KO3 ) to world-class boxers who went against him KO.
1991 was McLarnin induction into the International Boxing Hall of Fame.