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Mandel was born and raised in Toronto, Ontario, Canada, of Jewish descent. His father was a lighting manufacturer and realtor. After getting expelled from his high school (Northview Heights Secondary School) for impersonating a member of the school board and signing a construction company to make an addition to his school, Mandel became a carpet salesman who would later open a carpet sales business of his own. He became a stand-up comedian at Yuk Yuk's in Toronto and by September 1978 had a week-long booking as the featured act, billed as "a wild and crazy borderline psychotic." His repertoire including placing a latex glove over his head and inflating it by blowing through his nose, the fingers of the glove extending above his head like a cockscomb. When the audience reacted uproariously to that and similar antics, his trademark response was to extend his arms palms up, look incredulous, and ask, "What? What?" On a trip to Los Angeles, Mandel performed a set at The Comedy Store, which led to him becoming a regular performer there. A producer for the comedic game show Make Me Laugh saw him and booked Mandel for several appearances during the show's run in 1979. He was booked to open for David Letterman at shows in the summer of 1979. CBC-TV's head of variety programming saw a Mandel performance in October 1979 and immediately signed him for a TV special. In 1980, he won the lead role in the Canadian movie Gas, co-starring Susan Anspach and Donald Sutherland.

Mandel was the first "VeeJay" to appear on Nickelodeon's music video series, Pop Clips. That 1981 series would go on to provide the genesis of MTV.

Mandel tours on the road over 200 days a year, taping real or No real. He met his wife Terry in high school, and they were married in 1980. Has three children: Daughters Jackie (b. 1984) and Riley (b. 1992) and son Alex (b. 1989).

Mandel has obsessive-compulsive disorder (OCD) including enthusiastic contestants on Deal or No Deal, unless he is wearing latex gloves. Instead of shaking contestants' hands when they offer them, Mandel often opts to exchange fist pounds, put his hands on contestants' shoulders, or give an occasional hug. He once kissed a female contestant on the show for good luck despite his mysophobia. He now takes medicine to control his condition and even pokes fun at himself for it. He revealed on The Howard Stern Show on March 24, 2006 that his shaved head is not related to natural hair loss, but to his mysophobia. He stated that the lack of hair makes him feel cleaner. During his appearance on Mad TV, Mandel and cast member Bobby Lee made fun of the former's OCD. In September 2007, former NFL running back Marshall Faulk was interviewing with Howie on NFL Network asking him to shake his hands, but Howie wouldn't allow it; he'd rather have the fist tap, as he does with Deal or No Deal contestants. Seconds later, Faulk gave Howie a "sneak attack" shaking his right hand and Howie screamed in fear and walked away from Faulk. Howie washed his hands several times. On a guest appearance on Free Radio, Mandel pointed out that he wouldn't use any bathroom but his own, and sometimes brings bags with him. He warned not to approach him on the street if he is seen with a brown paper bag.

On September 4, 2008, Mandel received a star on the Hollywood Walk of Fame.

In October 2008, Mandel revealed that he has attention-deficit hyperactivity disorder (ADHD) in a series of public service announcements for the "Adult ADHD is Real" campaign.