Valentine began working on a casual basis for the World Wide Wrestling Federation, then owned by Vincent J. McMahon, in November 1978. Managed by The Grand Wizard, he was given the gimmick of a methodical wrestler who broke the legs of all his opponents, including Chief Jay Strongbow. In 1979, he faced WWF World Heavyweight Champion Bob Backlund and fought him to a one hour draw.
Valentine returned to the WWWF, now known as the World Wrestling Federation, for a year in 1981 and continued to pursue the title. On October 19, he was pinned by Backlund, but then handed the title belt by the dazed referee. The title was held up, and Backlund defeated Valentine cleanly in the November 23 rematch. He also feuded with Pedro Morales, who he "injured" by suplexing him on the concrete floor of the arena.
Valentine went to the WWF in 1984, predicting that Vince McMahon's plan for national expansion would succeed. His first manager was Captain Lou Albano, and he was later managed by Jimmy Hart.
On September 24, 1984 in London, Ontario Valentine defeated Tito Santana for the WWF Intercontinental Championship, focusing on Santana's injured knee throughout the match. Santana initially thought he had the match won, but had in fact only achieved a two-count. As Santana was celebrating, Valentine rolled him up for the pin and the title. Following the win, Valentine put Santana in the figure four leglock, reinjuring him. While Santana was sidelined having leg surgery, Valentine feuded with the Junkyard Dog, who he faced in an IC Title match at the very first WrestleMania held in Madison Square Garden. Valentine pinned the JYD after using the ropes, but Santana came to ringside and informed the referee, who restarted the match. Valentine then walked out, losing the match by count-out but saving his title. Santana eventually healed and fought Valentine in many matches (after Santana returned to the ring by defeating The Executioner in the first ever match at Wrestlemania). He went on to win the title back on July 6, 1985 in a steel cage match in Baltimore, ending Valentine's reign of more than 9 months. Valentine, incensed over losing the belt, destroyed it in the steel cage, forcing the WWF to get a new Intercontinental title belt.
Valentine would then form a tag team with Brutus Beefcake known as The Dream Team, managed by Johnny V. On August 24, 1985 the Dream Team defeated the U.S. Express (Barry Windham and Mike Rotunda) for the WWF World Tag Team Championship. They held the titles until April 7, 1986 when they were defeated at WrestleMania 2 by the British Bulldogs. At WrestleMania III, the Dream Team defeated Jacques Rougeau and Raymond Rougeau as a result of interference by Johnny V and Dino Bravo. Upset because of a missed move by Beefcake during the match, Valentine left with Bravo and abandoned Beefcake, resulting in Brutus turning face later on in the night.
Replacing Beefcake with Dino Bravo, Valentine formed The New Dream Team. Valentine was unhappy about teaming with Bravo, who he felt he had little chemistry with. After the WWF asked him to "kidnap" Matilda, the bulldog mascot of the British Bulldogs, Valentine resigned. He was brought back soon after, with Jimmy Hart as his new manager, but was used in a much diminished role between 1987 and 1990.
In 1988, Valentine feuded with newly-turned face Don "The Rock" Muraco, after Valentine viciously attacked Muraco's manager, former WWF World Heavyweight Champion "Superstar" Billy Graham. Graham, walking with a cane, tried to intervene when Valentine held the figure-four leglock on jobber Ricky Ataki, after already winning the match. Valentine then put the figure-four on Graham, who had a plastic hip.
Valentine was entered in the World Heavyweight Championship tournament at WrestleMania IV, where he defeated Ricky Steamboat before losing to Randy Savage, the eventual winner of the tournament. In April 1989 he began feuding with "Rugged" Ronnie Garvin. Two weeks after WrestleMania V, he defeated Garvin in a retirement match on Superstars of Wrestling by reversing a small package and holding on to the ropes. Garvin became a referee until Valentine and Hart managed to have him fired. Garvin then became a ring announcer, and began aggravating Valentine. At SummerSlam 1989 for his match with Hercules he announced Valentine as being Hercules' "so-called opponent" and at 249 lbs looking to Garvin like he was "overweight by thirty pounds" and as he approached the ring. Other 'insults' by Garvin during the introduction included Valentine not knowing whether he was "coming or going" and being the only wrestler Garvin knew who had "two left feet". Following the match Garvin then announced Hercules as the winner when Valentine cheated to win, despite the referee declaring Valentine the winner. Valentine knocked Garvin out of the ring before having a further punching match with Hercules while Garvin climbed back in the ring and nailed Valentine. The Hammer and Jimmy Hart eventually demanded that Garvin be reinstated so that they could fight in the ring.
For some time Valentine had been wearing a shin guard, which he would rotate (so it covered his calf, not his shin) in order to increase the pressure exerted by the Figure Four Leglock. He referred to the shin guard as the "Heartbreaker". Valentine would also (illegally) attach the shin guard to his arm in order to accentuate his elbow drops. In the course of the feud, Garvin countered with a rotated shin guard of his own, which he dubbed "the Hammer Jammer". The feud culminated in a submission match at the 1990 Royal Rumble. Garvin utilized the "Hammer Jammer" during the match, using it to "counter" Valentine's Figure Four Leglock when Valentine applied it on Garvin during the match. Jimmy Hart managed to remove the "Hammer Jammer" from Garvin's leg, after which Valentine systematically wore down Garvin's leg, but eventually lost after Garvin hit him with the Heartbreaker and applied a sharpshooter, forcing Valentine to submit.
Shortly after, formed a tag team with The Honky Tonk Man who was also managed by Jimmy Hart. For around eight months Valentine resisted dying his hair black (an idea of Jesse "The Body" Ventura) but eventually relented and the team became known as Rhythm and Blues and were managed by Hart. Rhythm and Blues feuded with The Bushwhackers and The Hart Foundation, but were overshadowed by the Legion Of Doom, who were by then dominating the tag division.
Valentine turned on Honky Tonk Man and became a face in December 1990. On January 19, 1991 Valentine participated in the fourth annual Royal Rumble match, where he lasted forty four minutes. He was to feud with his former partner, the Honky Tonk Man, but Honky left the WWF before the feud could begin. Valentine lost to Earthquake at WrestleMania VII, and was defeated again at SummerSlam 1991 by Irwin R. Schyster. He participated in the 1992 Royal Rumble, where he attacked Ric Flair before being eliminated by Repo Man.
He reappeared in the WWF at the 1993 Survivor Series under a mask as The Blue Knight in an elimination match pitting Shawn Michaels (who was substituting for Jerry Lawler) and his three "knights" (who also included Barry Horowitz as the Red Knight and Jeff Gaylord as the Black Knight) against the Hart Family. He would return again, as Greg Valentine, at the 1994 Royal Rumble, lasting over twenty minutes before being eliminated by Rick Martel.
On March 13, 2004, Valentine was inducted into the WWE Hall of Fame by his former manager Jimmy Hart. The following night, at WrestleMania XX at Madison Square Garden, Valentine received loud applause when the class of 2004 was introduced. Shortly after being inducted into the WWE Hall of Fame, Valentine dedicated the plaque he received to his late father by saying: "This one's for you pop, Johnny Valentine". On October 3, 2005, Valentine made an appearance at WWE Homecoming, and on October 23 he was defeated by Rob Conway (then using a gimmick similar to that of Randy Orton's gimmick of "the legend killer") on an episode of WWE Heat after Eugene interfered on his behalf, causing the referee to award a victory via disqualification to Conway. He was present at the Retirement Ceremony of his long-time friend, Ric Flair, on the March 31, 2008 episode of WWE Raw.