Kaplan is perhaps best known for his role as Gabe Kotter in the 1970s sitcom Welcome Back, Kotter.

As a boy Kaplan had aspirations of being a Major League Baseball player, however after two years he was unable to even make the roster of a minor league team and he decided to pursue other interests. He began working as a bellman at a hotel in Lakewood, New Jersey. Touring comedians would sometimes perform at the hotel, and Kaplan began to work towards a career as a comic.

Kaplan's comedy was successful, and he toured the country with his act based on his childhood experiences in Brooklyn. He appeared five times on The Tonight Show Starring Johnny Carson from May 1973 to December 1974. The sitcom Welcome Back, Kotter was in part based on his comedy act about the people he grew up with. Kaplan played a high school teacher named Gabe Kotter in the sitcom from 1975 to 1979. After Welcome Back, Kotter he continued with his stand up act and was in several movies.

While he was actively involved in his acting career, Kaplan also became involved in financial markets and poker. He made his first appearance at the World Series of Poker in 1978. His biggest win came in July 2004 when he finished 3rd at the World Poker Tour's no limit Texas hold 'em event, earning more than $250,000. He also finished 2nd at the 2005 World Series of Poker Limit Hold 'em event, winning $222,515. As of 2005 his career poker tournament earnings are in excess of $680,000.

Recently Kaplan has been doing stand up comedy and working on adaptations of Welcome Back, Kotter, but he still plays poker frequently and is a commentator for poker events and televised poker shows, most recently the National Heads-Up Poker Championship on NBC and High Stakes Poker on GSN.