Mero began playing football in his senior year, with his high school team winning the New York State Title under the tutelage of future University of Central Florida coach George O'Leary. In the same year, Mero began training as a boxer under Golden Gloves coach Ray Rinaldi. Mero went on to win four New York State titles, including the New York Golden Gloves tournament. Mero intended to become a professional boxer, but his career was sidelined after his nose was broken in an accident. He briefly pursued a bodybuilding career, placing third in the Mr. New York State bodybuilding contest.
In 1996, Mero left WCW because he strongly objected to an angle where he was associated with another man's wife (Kimberly Page). After leaving WCW he signed a contract with the World Wrestling Federation. Wrestling as "Wildman" Marc Mero. Mero was managed by his wife, Rena "Sable" Mero. On September 23, 1996, Mero defeated Faarooq Asad in the finals of a tournament for the vacant WWF Intercontinental Championship. He held the title until October 21, 1996, when he was defeated by Hunter Hearst Helmsley.
In February 1997, Mero tore his anterior cruciate ligament, and spent six months rehabilitating. He returned in late 1997 with the new persona of "Marvellous" Marc Mero, a boxer-turned-wrestler. The WWF storyline had Mero become jealous of Sable's fan following and wanting her out of the WWF. At Over the Edge, he defeated Sable with a small package, which resulted in her leaving the WWF for a short time. After he dropped Sable as his valet, Jacqueline became his manager.
In 1999, Mero and Sable left the WWF. At the time, Mero had three years remaining on his contract, with a guaranteed salary of $350,000 USD. Mero subsequently did not wrestle for 18 months due to various nagging injuries and a shoulder surgery.
On April 26, 2000, Mero returned to World Championship Wrestling, appearing with his trainer, Ray Rinaldi, in the audience on an episode of WCW Thunder and confronting Tank Abbott. Mero opted not to return to WCW on a full-time basis due to his physical condition at the time.
In 2001, Mero returned to wrestling in the short-lived X Wrestling Federation along with Rena Mero.
In November 2004, Mero began wrestling for Total Nonstop Action Wrestling as Johnny B. Badd. He appeared sporadically with TNA throughout late 2004 and early 2005. Mero retired in 2005, opening and operating the Marc Mero Body Slam Training Institute in Altamonte Springs, Florida.
In January 2008, WWE and Jakks Pacific announced that Mero would be featured in their "Classic Superstars" line of action figures.
Mero's comments attracted criticism from WWE employees such as Mr. Kennedy, who labelled him a "goof" and a "silly bastard", and Dave "Fit" Finlay, who claimed Mero had "nothing to do with this business". Kennedy himself was later suspended by WWE for purchasing steroids.