LaMarche was born in Toronto, Ontario, Canada, but his family moved to Timmins, Ontario, very soon after he was born. LaMarche's childhood was filled with his "own little world of cartoons and sixties television". It was not until his sophomore year of high school that he learned of the popularity his talent for mimicry could garner him. This realization came from a coincidental performance in a high school "variety night" when a couple of friends urged him to enter. The act he performed at the variety night was "celebrities as waiters" which he actually used all the way up until the end of his stand-up career.
At the age of 19, 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". This reaction was coupled with the backlash LaMarche received from fellow Canadian comedians who LaMarche describes as discouraging him from pursuing a career outside of Canada.
Three years later, at the age of 22, LaMarche moved straight to Los Angeles to further his stand-up career. This move, LaMarche says, would always be something he regretted doing instead of moving to New York: "... in retrospect, I thought it was a mistake. I think that a couple of years in New York would have made me a stronger comedian."
Over the next five years, LaMarche's career would gradually progress, playing comedy clubs all over the U.S., with several appearances on Merv Griffin and "An Evening At The Improv", but in spite of such interest, LaMarche always believed that, while his impersonations and stage presence were strong, he needed to develop funnier comedy material. Despite being so critical of himself, LaMarche would be granted the opportunity of being part of the 1985 HBO production, Rodney Dangerfield Hosts the 9th Annual Young Comedians Special, on which also appeared Bob Saget, Rita Rudner, Louie Anderson, Yakov Smirnoff, and the breakout first appearance of Sam Kinison. Although he was received (and reviewed) favorably, in looking back on his own performance in that special, LaMarche believed he was "probably about five years away from going from being a good comedian to being a great comedian" and being the "only impressionist that actually comes from somewhere".
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.
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. This sent LaMarche into depression and alcoholism for the next two years, effectively stalling his stand-up career. After getting sober on Inauguration Day in 1989, LaMarche embarked again into the world of his first love, standup comedy, in the early part of 1990. However, just as he was regaining lost momentum, tragedy struck once more, as his 18-year-old sister was killed in a car accident in September of that year. At this point, though he remained sober, LaMarche decided he just could not do standup comedy anymore. He says, "at that point I just threw up my hands and went, 'Oh, that’s it. I don’t have any funny left in me. I’m done.'"
- One of his characters, Mr. Mandrill has the same first name as him.