[6] At the age of nineteen, LaMarche took his high school act to an open mic night in New York City, performing to a reaction in which, as he describes, "they just totally ignored me".

"[9] Over the next five years, LaMarche's career progressed, playing comedy clubs throughout the US, with several appearances on The Merv Griffin Show and An Evening At The Improv.

In spite of such interest, LaMarche believed that, while his impersonations and stage presence were professional, he needed to develop funnier comedy material.

[10] During his standup career, LaMarche opened for such acts as Rodney Dangerfield, George Carlin, Howie Mandel, David Sanborn, and Donna Summer, usually in the main showrooms of Las Vegas and Atlantic City.

[11] On March 9, 1987, LaMarche's father was murdered, shot to death by a lifelong friend in a Toronto hotel lobby in front of dozens of witnesses.

[13] LaMarche's entry into the voiceover industry was in 1980 in Easter Fever and Take Me Up to the Ball Game, two animated Canadian TV specials from Nelvana.

[14] He also was a regular voice performer on Toronto's pioneering cult TV hit The All-Night Show, which debuted in September 1980; a continuing feature had the lips of LaMarche inserted into a photograph of a famous person, and having LaMarche imitate that person to deliver a show promo or announcement.

[15] It took a few years after his 1981 move to Los Angeles for LaMarche to establish himself in the American voiceover industry, which he initially pursued as a side endeavor during his full-time standup comedy career.

LaMarche began on Inspector Gadget with that show's second season in 1985, and went on to Dennis the Menace, Popeye and Son and The Real Ghostbusters where he played Egon Spengler.

After The Real Ghostbusters, LaMarche became a regular mainstay of the voiceover industry, appearing in such shows as Tiny Toon Adventures, GI Joe, Attack of the Killer Tomatoes: The Animated Series, Taz-Mania, Where's Waldo, The Little Mermaid, Batman: The Animated Series, and Bonkers before landing perhaps his most recognized role in 1993 as The Brain on Animaniacs and later its spin-off show Pinky and the Brain.

He also played the voice role of Grumpy in the 2014 Disney XD series The 7D, a reimagining of Snow White and the Seven Dwarfs.

He also delivered the protracted belches for the "Great Wakkorotti" shorts on Animaniacs, in which Wakko Warner performed various pieces of music.

LaMarche acted, voice only, in the second episode of the hit NBC show Heroes, "Don't Look Back", as the villain Sylar.

[22] LaMarche appeared in many films, including dubbing the voice of Orson Welles over Vincent D'Onofrio's on-camera performance in Ed Wood; Pepé Le Pew in Space Jam; supplying the voice of the Alec Baldwin puppet in Team America: World Police, and reprising his roles from Queer Duck and Futurama in the direct-to-video films Queer Duck: The Movie and Futurama: Bender's Big Score, respectively.

His one on-camera theatrical film performance was in the 1981 Canadian feature Funny Farm, not to be confused with a later Chevy Chase vehicle of the same name.

In Mark Hamill's 2004 film Comic Book: The Movie, LaMarche made a rare live appearance to be in the special features of the DVD alongside Pinky and the Brain co-star Rob Paulsen.