[8] After his first divorce, Belzer relocated to New York City, moved in with singer Shelley Ackerman, and began working as a stand-up comic at Pips, The Improv, and Catch a Rising Star.

He participated in the Channel One comedy group that satirized television and became the basis for the cult movie The Groove Tube, in which Belzer played the co-star of the ersatz TV show The Dealers.

A short skit of a younger Belzer can be found on Sesame Street in a season 9 episode in 1978 when two young men attempt a picnic and boat ride, only to be thwarted by a dog who eats their food.

[11] In addition to his film career, Belzer was a featured player on the National Lampoon Radio Hour with co-stars John Belushi, Chevy Chase, Bill Murray, Gilda Radner, and Harold Ramis, a half-hour comedy program aired on 600 plus U.S. stations from 1973 to 1975.

[12] Several of his sketches were released on National Lampoon albums, drawn from the Radio Hour, including several bits in which he portrayed a pithy call-in talk show host named "Dick Ballantine".

[11] He followed that with starring roles on the Baltimore-based Homicide: Life on the Street (1993–1999) and the New York City-based Law & Order: Special Victims Unit (1999–2013), portraying police detective John Munch in both series.

[4] Barry Levinson, Executive Producer of Homicide, said Belzer was a "lousy actor" in audition when he read lines from the script for "Gone for Goode", the first episode in the series.

[19] These shows were on six different networks: In March 2016, executive producer Warren Leight announced Belzer would return to reprise the role in a May 2016 episode of Law & Order: Special Victims Unit, titled "Fashionable Crimes".

On June 9, 2001, Belzer himself was honored by the New York Friars Club and the Toyota Comedy Festival as the honoree of the first-ever roast open to the public.

[34] His 1997 HBO special and comedy CD Another Lone Nut pokes fun at this medical incident, as well as his status as a well-known conspiracy theorist.

On March 27, 1985, four days before the first WrestleMania, Belzer repeatedly requested on his TV talk show Hot Properties that professional wrestler Hulk Hogan demonstrate a wrestling move.

Hogan applied a front facelock, which caused Belzer to pass out, and he hit the back of his head on the floor when released.