In 1993, Nash signed a contract with the World Wrestling Federation at the request of Shawn Michaels. He was given the stage name of "Diesel", with the character given the nickname "Big Daddy Cool". Taking on the appearance of a typical biker thug, Diesel sported black sunglasses and leather garments.
Diesel started out as the bodyguard/best friend of Shawn Michaels. He made his WWF debut at a house show on June 6, 1993, assisting Michaels in defeating Marty Jannetty for the Intercontinental Championship. In January 1994, Diesel appeared at the Royal Rumble, eliminating seven men in under 18 minutes of in-ring time. Diesel won the Intercontinental Championship from Razor Ramon, following interference from Michaels on the April 13, 1994 edition of Superstars. The duo of Diesel and Michaels defeated The Headshrinkers to win the WWF Tag Team Championship on August 28, making Nash a double champion. The next night, Ramon defeated Diesel at SummerSlam to regain the Intercontinental Championship. The alliance between Diesel and Michaels dissolved after Survivor Series after Michaels accidentally superkicked Diesel. Diesel then gave chase to Michaels, and although he failed to catch him the reaction from the crowd turned him babyface. However, Nash was no longer a tag team champion, as Michaels' actions resulted in the team being forced to vacate the titles.
On November 26, three days after Survivor Series, Diesel faced Bob Backlund for the WWF Championship he won at Survivor Series from Bret Hart. In the match, held at Madison Square Garden, Diesel defeated Backlund in an eight-second squash match. Diesel then promised Hart a match for his title, which they had at the 1995 Royal Rumble. The match ended in a draw due to interference from several wrestlers, including Shawn Michaels. Michaels was irate about his former bodyguard having beaten him to the WWF Championship and was sufficiently motivated to win the 1995 Royal Rumble later that evening, earning himself a title shot at WrestleMania XI.
At WrestleMania XI on April 2, Nash, accompanied to ringside by actress Pamela Anderson (who was supposed to valet for Michaels), defeated Michaels to retain the title. After the match he left the ring with both Anderson and Michaels' replacement for her, Jenny McCarthy. The next night on Monday Night Raw, Michaels was betrayed by his new bodyguard, "Sycho" Sid, prompting Diesel to come to his rescue. This occurrence led to the tag team being reunited, now known as "Two Dudes with Attitude". Diesel successfully defended the WWF title against Sycho Sid at the inaugural In Your House pay-per-view in Syracuse, New York, and at the In Your House 2: The Lumberjacks in Nashville, Tennessee. At SummerSlam 1995, Diesel retained the WWF title by defeating Mabel, who had won the 1995 King Of The Ring tournament.
On September 24 at In Your House 3 in Saginaw, Michigan, Diesel and Michaels challenged the reigning WWF Tag Team Champions Owen Hart and Yokozuna. The match had a winner-take-all stipulation, as in addition to the tag belts Diesel's WWF Championship and Michaels' recently won Intercontinental Championship were also on the line. When Hart did not show up at the event, he was replaced by Davey Boy Smith. In the course of the match, Hart arrived at ringside, entered the ring, and was pinned by Diesel for the win and the title, making him and Michaels holders of all three major WWF championships. The reign did not last long, however, as Hart and Yokozuna had the titles returned to them the next night due to Hart not being an official part of the match when pinned.
Diesel's WWF Championship reign continued until November 19, when he was defeated by Bret Hart at the Survivor Series. Following the loss, Diesel attacked Hart. He later began a tweener turn and started a rivalry against fan favorite the Undertaker by costing him a potential victory in his match against Hart at the 1996 Royal Rumble. Diesel was the last wrestler eliminated in the Royal Rumble match by Shawn Michaels, which eventually led to a rift between the two men. At In Your House 6 in Louisville, Kentucky Diesel lost his steel cage match against WWF Champion Bret Hart when he was attacked by the Undertaker.
Diesel lost to The Undertaker at WrestleMania XII on March 31, then he finally turned heel and went on to feud with Shawn Michaels once again after he turned on him at a Madison Square Garden live event. In his last televised WWF appearance until 2002, Diesel challenged Michaels for the WWF Championship (which he had won from Hart at WrestleMania XII) at In Your House: Good Friends, Better Enemies on April 28. He wrestled Michaels for the title once again in a steel cage match at a house show on May 19, but was again defeated. After the match, Diesel, Michaels, Razor Ramon, and Hunter Hearst Helmsley, a group of off-screen friends known collectively as "The Kliq", hugged one another in the ring and wished each other farewell. This incident, later referred to as the "Curtain Call" or "MSG Incident". He and Razor Ramon left thereafter.
As the nWo, Nash and Ramon (Scott Hall) (and Hulk Hogan) returned to the WWF at No Way Out on February 17, 2002. In the course of the evening, the nWo delivered an interview in which they gave a six pack of beer to Stone Cold Steve Austin (which he refused) and traded insults with The Rock. They interfered in the main event of the evening, helping Chris Jericho retain his Undisputed WWF Championship against Austin. At WrestleMania X8, Nash continually interfered in the match between Hall and Austin to the point where he was forced to return backstage. Later that night, he and Hall turned on Hogan after he had offered congratulations to The Rock for defeating him. In March, Nash suffered a biceps injury that put him out of action for several weeks and almost immediately upon returning, suffered a quadriceps tear in a tag match on the July 8 episode of Raw. On the July 15 edition of Raw, the nWo was officially disbanded by Vince McMahon as Eric Bischoff became Raw General Manager.
After nine-month injury, Nash returned as a face on the April 7, 2003 episode of Raw, much to the delight of both Shawn Michaels and Triple H, who were feuding with each other. As part of the storyline, Nash was given a choice to remain friends with either Michaels or Triple H. After Nash would not make the decision, Triple H made the decision for him and turned on him with a low blow. This led to Nash and Triple H feuding with one another. Nash teamed up with Michaels and Booker T against Triple H, Ric Flair and Chris Jericho in a Six-man tag team match at Backlash which ended in Triple H picking up the win for his team, pinning Nash after hitting him with a sledgehammer. Following Backlash, Nash was granted a shot at Triple H's World Heavyweight Championship, and the two squared off at Judgment Day with Michaels and Flair in their respective corners. Triple H would get himself disqualified and kept the title as a result, but this did not stop Nash from attacking Triple H following the match, putting him through the announcer's table with a Jackknife Powerbomb. The next month, they fought again in a Hell in a Cell match at Bad Blood with Mick Foley as the special guest referee but Nash lost the match.
In August 2003, Nash feuded with Chris Jericho and was forced to cut his hair after losing a hair vs. hair match against Jericho. His last match in WWE was at SummerSlam in an Elimination Chamber match for the Heavyweight championship against Triple H, Shawn Michaels, Goldberg, Jericho, and Randy Orton. He was the first to be eliminated after Jericho pinned him following Sweet Chin Music from Michaels. Before leaving however, he executed a Jackknife Powerbomb on Jericho and Orton. Nash then stepped away from in-ring action and underwent neck surgery. The WWE decided not to renew Nash's contract in January 2004.
On January 30, 2011, at the Royal Rumble, Nash, billed as Diesel for the first time since 1996, returned to the promotion, taking part in the Royal Rumble Match. He entered the match as number 32 and was eliminated by Wade Barrett. It was announced that he had signed a five-year WWE Legends contract. On April 2, Nash, along with Sean Waltman, was on hand to celebrate the induction of Shawn Michaels into the 2011 WWE Hall of Fame. Triple H inducted Michaels, and after Michaels gave his speech, Nash and Waltman joined the two on stage to celebrate.
Nash, no longer billed as Diesel, returned at SummerSlam in August, attacking CM Punk after he became Undisputed WWE Champion turning heel, which allowed Alberto Del Rio to cash in his Money in the Bank briefcase and win the title. The following night on Raw, Nash claimed Triple H, on-screen Chief Operating Officer of WWE, had instructed him by text to attack the winner. Punk verbally berated Nash on the microphone, so Nash attacked him the next week. He also distracted Punk in a match, making him miss out on a championship match.
Nash was signed to an on-screen contract the next week by John Laurinaitis and demanded a match against Punk. After Triple H put himself in the match against Punk instead, Nash assaulted them both at a contract signing thus turning heel and was fired on screen. At Night of Champions, Nash interfered in their match, alongside The Miz and R-Truth. Triple H would then attack Nash with a sledgehammer before winning the match. Nash returned at the following pay-per-view, Vengeance to help Miz and Truth beat Punk and Triple H. After the match he jacknife powerbombed Triple H and attacked him again the following night with Triple H's sledgehammer, preventing him from receiving medical attention and taking him off TV. The following Monday, October 31, 2011 Laurinaitis once again signed Nash to a new contract.
He would continue to appear on Raw after the assault on Triple H, delivering his Jacknife finisher to Santino Marella and cutting promos about how Triple H was more of a boss than a friend. On the December 5, 2011 episode of Raw, Nash competed in his first televised WWE match in eight years, defeating Santino Marella. Nash went on to face Triple H at the TLC: Tables, Ladders & Chairs in a Ladder Match with a sledgehammer hanging above the ring which he lost by pinfall after receiving a sledgehammer shot to the face, ending the feud in the process.
In late 2012 and early 2013, Nash would begin appearing on WWE's developmental training show, WWE NXT. Nash initially appeared as the guest Match Commissioner for the night, a title given to him at the request of Dusty Rhodes. After announcing this to the crowd Nash was interrupted by Heath Slater to whom he promptly delivered a powerbomb, effectively turning face. Nash later again appear on WWE NXT to reunite with members of the Kliq, allowing him to effectively settle all issues with Triple H, embracing him once again. The reunion also turned into a reunion of D-Generation-X and Nash was awarded the title of being an honourary member of the stable. Nash then helped the Kliq take down Damien Sandow.
Nash competed in the 2014 Royal Rumble match as the 14th entrant, eliminating Jack Swagger before being eliminated by Roman Reigns. On February 20, 2014 it was announced that Nash would be on the NXT Arrival pre-show panel, on the WWE Network. On April 6, 2014, Nash inducted former tag team partner and long-time friend Razor Ramon into the WWE Hall of Fame. He appeared on WWE Raw on August 11th 2014 to reunite with Scott Hall and Hulk Hogan to celebrate Hulk Hogan's birthday.