Mero first appeared with WCW as an enhancement talent for several months in 1991. Mero competed in his first match on the May 1, 1991 episode of WorldWide, where he was defeated by Sid Vicious. He was soon repackaged with the ring name Johnny B. Badd and he was given the character of a flamboyant Little Richard look-alike. He was initially a villain and managed by Teddy Long. He debuted at the inaugural SuperBrawl pay-per-view, on May 19, 1991, where he was introduced as Long's newest client. He made his televised in-ring debut on the June 1 episode of World Championship Wrestling against enhancement talent Kip Abee. Badd made his pay-per-view debut at The Great American Bash, where he lost to The Yellow Dog by disqualification after Long tried to remove Yellow Dog's mask.
Badd was heavily pushed upon his debut, being placed in the rankings of the top ten contenders for the World Heavyweight Championship due to a dominant undefeated streak. He put his lip stickers on the faces of his defeated opponents after matches. On August 25, he participated in a tournament for the vacated United States Heavyweight Championship, where he faced Yellow Dog to a no contest in the quarter-final. As a result, both men were eliminated from the tournament. Badd suffered his first pinfall loss on television on the September 5 Clash of the Champions XVI: Fall Brawl, where he was pinned by the United States Heavyweight Champion Sting. Badd continued his dominance with a win over Jimmy Garvin at Halloween Havoc and unsuccessfully challenged Brian Pillman for the Light Heavyweight Championship on the November 19 Clash of the Champions XVII.
On the December 22 episode of Main Event, Badd teamed with Diamond Studd to take on Tom Zenk and P.N. News in a tag team match, which Badd's team lost. After the match, Badd was attacked by Studd and Diamond Dallas Page until Zenk and News came to his rescue. As a result, Badd turned into a fan favorite. Badd made his first appearance at the company's flagship event Starrcade, where he was randomly placed with Arachnaman in a Lethal Lottery tag team match against Scott Steiner and Firebreaker Chip; Badd's team lost. Shortly after, he began using "Badd Blaster", a confetti gun that he would fire before his matches. Badd continued to rack up wins against the likes of Richard Morton and Tracy Smothers in the first half of 1992. He feuded with Page and his teammate Scotty Flamingo and the two competed in a series of televised and non-televised matches throughout 1992, including a boxing match on the November 18 Clash of the Champions XXI, which Flamingo won in the third round after Page filled Flamingo's boxing glove with water.
Badd got involved in a rivalry with Cactus Jack after the two were randomly placed as tag team partners via Lethal Lottery for Starrcade, but failed to get along. The duo teamed for the first time on the December 26 episode of Saturday Night, where they defeated Tex Slazenger and Shanghai Pierce. They had a confrontation after the match, during which Jack delivered a double underhook DDT. At Starrcade, Badd's team lost in their Lethal Lottery match after Badd knocked Jack out with a Tutti Frutti. Badd faced Jack in a match on the January 13, 1993 Clash of the Champions XXII, where he lost. Badd began his next major program with Maxx Payne after Payne injured him with a Payne Killer on February 9. During this rivalry, Payne stole the Badd Blaster and Badd attempted to reclaim his gun. Badd lost a match to Payne by forfeit on the June 16 Clash of the Champions XXIII after Payne took him out with the Badd Blaster. Badd donned a mask due to the sustained injuries and faced Payne in a series of matches. He defeated Payne at the Beach Blast pay-per-view, but lost to him in a no disqualification match on the July 31 episode of Saturday Night. Badd defeated Payne in a mask versus guitar match on the August 18 Clash of the Champions XXIV, to claim Payne's guitar. The two ended their feud on the October 30 episode of Saturday Night, where both men shook hands and buried the hatchet after a confrontation with Paul Orndorff.
Badd unsuccessfully challenged Lord Steven Regal for the WCW World Television Championship on the November 10 Clash of the Champions XXV. He was randomly paired with Brian Knobbs in a Lethal Lottery at the Battlebowl pay-per-view, where Badd's team won to qualify for the Battlebowl battle royal. He eliminated Rip Rogers before being eliminated by Paul Orndorff.
Badd received his first world championship opportunity on the February 12, 1994, episode of Saturday Night, where he challenged Rick Rude for the International World Heavyweight Championship in a losing effort. During this time, Badd formed a tag team with Michael Hayes and began a feud with Harlem Heat. Hayes turned on Badd during a no disqualification match against Harlem Heat on the February 19 episode of Saturday Night. As a result, Badd would be facing Hayes, the following night at SuperBrawl IV, where Hayes was replaced by Jimmy Garvin, whom Badd defeated. Badd began pursuing the United States Heavyweight Championship after he lost a tuxedo match against champion Steve Austin on the May 7 episode of WorldWide, after Col. Robert Parker pulled off Badd's pants. Badd faced Austin for the title in a losing effort at Slamboree. Badd received another title shot on the June 23 Clash of the Champions XXVII, where Austin initially retained the title by pinning Badd after hitting him with a foreign object, but the referee reversed the decision, during which Badd pinned Austin. However, due to the controversial finish, Austin retained the title.
Badd substituted for the injured Sting as Lord Steven Regal's challenger for the World Television Championship at Bash at the Beach, where he failed to win the title. However, Badd remained in the title contention. Badd beat Regal for the title at Fall Brawl to gain his first singles championship in WCW. Badd made his first successful title defense against Bobby Eaton on the October 1 episode of WorldWide. He retained the title in major title defenses against The Honky Tonk Man at Halloween Havoc and the November 16 Clash of the Champions XXIX. Badd was scheduled to defend the title against Honky for a third time at Starrcade, but Honky left the company before the event and was replaced by Arn Anderson, whom Badd defeated to retain the title. Badd dropped the title to Anderson on the January 8, 1995, episode of Main Event. Badd failed to win the title in subsequent rematches on the January 25 Clash of the Champions XXX, February 18 episode of Saturday Night and February 19 episode of Main Event. Badd defeated Anderson in a boxer-versus-wrestler match at Uncensored to end the rivalry.
Badd competed in the mid-card throughout 1995, until he defeated Brian Pillman to become the number one contender for the United States Heavyweight Championship at Fall Brawl. Badd received his title shot on the September 30 episode of Saturday Night against Sting, but failed to appear as he had been attacked by the Television Champion Diamond Dallas Page. Badd faced and defeated Page to win his second World Television Championship at Halloween Havoc. Badd defended the title against Page at the inaugural World War 3 pay-per-view, with the stipulation that if Badd won, he would gain Page's Diamond Doll as his valet. Badd defeated Page to retain the title and won the services of Kimberly, but he freed her. She continued to be in Badd's corner and the rivalry with Page continued. Badd defeated Page to retain the title at SuperBrawl VI in 1996. Badd lost the World Television Championship to Lex Luger during a house show on February 17, but defeated Luger to regain the title on February 18. He lost the title to Luger again on the March 9 episode of Saturday Night, after Page delivered a Diamond Cutter to Badd on the floor. This was Badd's final appearance in WCW, as he left the company soon thereafter.
After leaving WCW, he immediately signed a contract with the World Wrestling Federation (WWF), where he began competing under his real name, Marc Mero. He debuted in WWF as a fan favorite at WrestleMania XII, on March 31, 1996, where he rescued his wife, Sable, from Hunter Hearst Helmsley. Mero made his in-ring debut, the following night on Monday Night Raw, where he defeated Isaac Yankem DDS. Beginning with the April 15 episode of Raw, Sable began accompanying Mero to the ring during his matches and he adopted the nickname "Wildman". Mero defeated Helmsley in his pay-per-view debut at In Your House 8. Mero qualified for the King of the Ring tournament by defeating Skip on the June 10 episode of Raw. He defeated Owen Hart in the quarter-final, the following week on Raw, but lost to eventual winner Stone Cold Steve Austin in the semi-final at King of the Ring. Mero began a rivalry with Goldust after Goldust won a match between the pair on the July 1 episode of Raw. Mero faced Goldust in a losing effort at the SummerSlam pay-per-view.
On the September 6 episode of Raw, Mero participated in a tournament for the Intercontinental Championship, which was vacated by Ahmed Johnson due to injury. Mero defeated Stone Cold Steve Austin, Owen Hart and Faarooq to win the title on September 23 episode of Raw. Mero successfully defended the title against Goldust, with the assistance of Mr. Perfect, at In Your House 10. He dropped the title to rival Hunter Hearst Helmsley, the following night on Raw after Perfect goaded Mero into defending the title against Helmsley and hit him with a steel chair. Mero lead a team of Jake Roberts, The Stalker and newcomer Rocky Maivia against Helmsley, Crush, Goldust and Jerry Lawler in a four-on-four elimination tag team match at Survivor Series. Mero was eliminated by Crush but his team won the match. Mero faced Helmsley for the Intercontinental Championship at In Your House 12, where Mero won by count-out.
Mero participated in the 1997 Royal Rumble match as the number sixteen entrant. He was eliminated by the eventual winner Stone Cold Steve Austin. On the February 3 episode of Raw, Mero received his last opportunity for the Intercontinental Championship against Hunter Hearst Helmsley, where he failed to win the title. Later that month, he tore his anterior cruciate ligament, and spent six months rehabilitating.
Mero returned to WWF television on the October 6, 1997 episode of Raw is War, where he was repackaged as a boxer-turned-wrestler and got a new nickname "Marvelous". He debuted a new finishing move, TKO, to win his match against Miguel Pérez, Jr.. Mero began a slow turn as a villain after becoming jealous of the fan following Sable had acquired in his absence and wanting her out of the WWF. During this time, he faced boxer Butterbean in a toughman contest at D-Generation X: In Your House, which he lost by disqualification.
As 1998 began, Mero's obsession with Sable continued, which led to a temporary alliance with Goldust and his valet, Luna Vachon, in which the three humiliated Sable during matches, but it turned into a short feud when Goldust became physical with Sable. At WrestleMania XIV, Mero and Sable defeated Goldust and Luna in a mixed tag team match. The storyline was resolved in a match between Mero and Sable at Over the Edge, in which Mero feigned remorse, offering to lie down and allow her to pin him, before defeating Sable with an inside cradle, which resulted in her leaving the WWF for a short time and Mero completed his villainous turn. After he dropped Sable as his valet, Jacqueline became his manager. However, his association with Jacqueline did not prove to be successful as he failed to qualify for the King of the Ring tournament against Jeff Jarrett on the June 22 episode of Raw is War and competed in the Brawl for All on the following week's episode of Raw is War, where he was defeated by Steve Blackman in the first round. Mero and Jacqueline faced Sable and newcomer Edge in a mixed tag team match at SummerSlam, which Mero lost after Edge dropped Sable on him to pin him for the victory. Mero's misfortunes continued when he failed to regain the Intercontinental Championship after losing to Val Venis in a tournament in October. Jacqueline's interventions in matches soon ended up costing Mero several victories and he ultimately fired her after losing to Big Boss Man on the November 22 episode of Sunday Night Heat.
Mero's last appearance on WWF television in the United States was on the November 30 episode of Raw is War, where he had a shot at the Light Heavyweight Championship against Duane Gill. He promised to retire if he did not win the title; Gill won the match with the help of The J.O.B. Squad. He wrestled one final time at the United Kingdom-exclusive pay-per-view Capital Carnage, where he teamed with Jacqueline as a reluctant partner against Sable and Christian in a losing effort. In early 1999, both Mero and Sable left the WWF.
On April 26, 2000, Mero returned to World Championship Wrestling, appearing in the audience on an episode of WCW Thunder and confronting Tank Abbott.