Kaplan and his family emigrated to the United States from the Russian Empire when he was five years old and settled in Meriden, Connecticut. While selling fruit for five cents a day, he began boxing as a teenager at the Lenox Athletic Club in Meriden, and turned professional in 1919.
Kaplan began his boxing career in the Connecticut State circuit. At the time, Meriden was an epicenter of boxing in the northeastern United States. Early in his career, Kaplan fought against the popularity of local favorite and former New York State champion Charlie Pilkington. Though they never boxed a professional match against each other, this early rivalry and Pilkington's role as a mentor and sparring partner had much to do with launching Kaplan's very successful boxing career.
A busy fighter, he engaged in over 50 bouts in his first four years in the paid ranks. During one of his early fights in 1920, Kaplan knocked out opponent Sailor Cunningham within two minutes.
After winning against Hughie Hutchinson and Earl Baird in 1922, Kaplan gained significant popularity.
In 1923 he twice drew with rival Babe Herman before scoring a 10-round win over future world lightweight champ Jimmy Goodrich.
By late 1924, World Featherweight champion Johnny Dundee vacated his title and a tournament was arranged to determine a successor. "Kid" kayoed Angel Diaz in three rounds, outpointed Bobby Garcia over 10-rounds and then halted Joe Lombardo in four rounds to advance to the finals. On January 2, 1925 he knocked out Danny Kramer in nine rounds at Madison Square Garden to become the new champion. His first two defenses were against the familiar Babe Herman (D15 and W15) in late 1925. Kaplan next decisioned Hall of Famer Billy Petrolle over 12 rounds in a non-title bout.
However, Kaplan's reign as champion was nearing its end. Despite standing 5 ft 4 in., he was experiencing difficulty making the featherweight limit and decided to relinquish the crown to campaign as a lightweight in 1927. As a 135-pounder, he scored wins over Jackie Fields, Johnny Jadick, Billy Wallace, Battling Battalino and Sammy Mandell among others. Amongst the wins were loses to Wallace and Hall of Famer Jimmy McLarnin. In 1933 he lost to Herbert "Cocoa Kid" Lewis Hardwick, and promptly retired from the ring with a 108-17-13D- 12 ND (26KOs) record.
Known as a rugged, pressing boxer who possessed tremendous stamina, the crowd-pleasing "Meriden Buzzsaw" died on October 26, 1970 in Norwich, CT.
Kaplan was inducted into the International Boxing Hall of Fame in 2003.
Kaplan, who was Jewish, was inducted into the International Jewish Sports Hall of Fame in 1986.Brother of fellow boxer Israel "Izzy" Kaplan