Nash debuted in World Championship Wrestling (WCW) as the orange-mohawked "Steel," one half of the tag team known as The Master Blasters. He was initially partnered with Master Blaster Iron, with whom he made his debut at Clash of the Champions XII on September 5, 1990, defeating Brad Armstrong and Tim Horner. At the following Worldwide taping on September 7, the Masters Blasters began a feud with Tim Horner and Mike Rotundo. They would defeat Horner and Rotundo on several house shows. On September 22, Nash's partner Master Blaster Iron was replaced by "Blade."
The reconstituted Master Blasters continued their undefeated streak in October. Meanwhile, Nash would have his first singles match on September 28 by defeating Tom Zenk. At Halloween Havoc on October 27, 1990, the Blasters upended The Southern Boys and began to move up the WCW tag team ratings. However, their winning streak would finally come to an end on November 22, when Tom Zenk and Brian Pillman handed them their first defeat with Pillman pinning Blade. They rebounded to go on another undefeated streak by defeating The Southern Boys as well as Alan Iron Eagle and Tim Horner, and earning a WCW United States Tag Team Championship title shot against then-champions The Steiner Brothers, but were defeated in two occasions. They were squashed on television in 52 seconds by the Steiners in a match that aired on Worldwide on February 2, 1991. Following this loss, their momentum began to dissipate as the Blasters would suffer follow-up losses to The Southern Boys and Ricky Morton and Tommy Rich. The tag team disbanded at the end of the month.
Nash was then rebranded as simply The Master Blaster in February 1991 and suffered his first singles defeat on February 27, 1991 when he was pinned by The Junkyard Dog. He also lost to Brian Pillman in house show matches, while appearing in tag team matches with Stan Hansen and Arn Anderson. His final match in this guise was against Pillman at a house show on May 12.
A week later, Nash reappeared under his new gimmick, having been repackaged as the silver-haired Oz. Oz, managed by The Great Wizard, was pushed strongly for about a month; he squashed several wrestlers before losing to Ron Simmons at The Great American Bash on July 14. On October 27, he lost to Bill Kazmaier at Halloween Havoc. Nash wrestled as Oz throughout the remainder of 1991 and went on a lengthy losing streak, suffering defeats by Kazmaier, Rick Steiner, Dustin Rhodes, and Arachnaman.
On January 21, 1992, at Clash of the Champions XVIII, he was repackaged as Vinnie Vegas, a wisecracking pseudo-mobster. Vegas was quickly recruited into "A Half-Ton of Holy Hell," a stable of large wrestlers created by Harley Race which included Big Van Vader and Mr. Hughes. The stable separated in February 1992, and Vegas joined The Diamond Mine, a stable led by Diamond Dallas Page that also included The Diamond Studd and Scotty Flamingo. After Studd and Flamingo left the stable, Page and Vegas began teaming together as The Vegas Connection. The tag team split in late 1992 after Page was fired. Nash spent the first half of 1993 teaming with Big Sky. In June, he decided to depart for the World Wrestling Federation (WWF) and had his final WCW match was on June 3, teaming with Big Sky in a losing effort against The Cole Twins; this match would air on Worldwide after his WWF debut.
In 1993, Nash left WCW, signing a contract with the WWF. He was given the name Diesel, with the character given the nickname "Big Daddy Cool." For the role, Nash grew long hair, taking on the appearance of a typical cocky biker thug and sporting black sunglasses and leather garments.
Diesel started out as the bodyguard/best friend of Shawn Michaels, with the two being known as Two Dudes with Attitudes. He made his WWF debut at a house show on June 6, 1993 by assisting Michaels in defeating Marty Jannetty for the WWF 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 30, 1994 episode (taped April 13, 1994) 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. However, Diesel lost the Intercontinental Championship back to Ramon the following night at SummerSlam. The alliance between Diesel and Michaels dissolved after Survivor Series, when Michaels accidentally performed a superkick on Diesel. Diesel then chased Michaels, and despite failing 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 World Heavyweight 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 the following month at the 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 World Heavyweight Championship and was sufficiently motivated to win the Royal Rumble match 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 and thus reunited the tag team. Diesel successfully defended the WWF World Heavyweight Championship against Sycho Sid at the inaugural In Your House pay-per-view on May 14, and at In Your House 2: The Lumberjacks on July 23. At SummerSlam, Diesel retained the WWF World Heavyweight Championship by defeating Mabel, who had won the 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 World Heavyweight 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 on Raw due to Hart not being an official part of the match when he was pinned.
Diesel's WWF World Heavyweight Championship reign continued until November 19, when he was defeated by Bret Hart at 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 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 on February 18, Diesel lost a steel cage match for the WWF World Heavyweight Championship against Bret Hart when he was attacked by The Undertaker.
Diesel lost to The Undertaker at WrestleMania XII on March 31, after which 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", was a serious breach of character, as it showed heels and babyfaces consorting with one another. Shortly thereafter, with his obligations to the WWF now completely fulfilled, Nash left for WCW.
After two weeks of Scott Hall (Razor Ramon) returning on WCW programming and taunting announcers, wrestlers, and the company, Nash also returned alongside his friend on June 10, 1996, after Hall interrupted Eric Bischoff. The duo were known as The Outsiders, and the storyline originally pushed them as "invaders" from the WWF. At Bash at the Beach, Hall and Nash fought the team of Lex Luger, Sting, and Randy Savage and promised to add one more man to their entourage. After Luger was taken out of the match, Hulk Hogan came out to make the save, only to turn on Savage and reveal himself as Nash and Hall's third man. Immediately after this, they began cutting promos calling themselves the New World Order (nWo). During his return, he had dyed his hair blonde. Through late 1996 and into 1997, Nash normally teamed with Hall as the Outsiders, and they held the WCW World Tag Team Championship. Nash also began to show his leadership qualities in the nWo, and became a sort of "second in command" alongside Hogan. Nash, Hall, and Syxx distinguished themselves from the rest of the nWo, calling themselves the "Wolfpac" in 1997.
After a while, however, the nWo began to fight within its ranks, with Hogan and Nash battling for control. The situation came to a head on April 20, 1998, during a match between Hogan and recent nWo inductee (and rival) Randy Savage for Savage's recently won WCW World Heavyweight Championship. During the match, Nash interfered on Savage's behalf and jackknifed Hogan to the mat, signaling the breakup of the nWo into two separate factions (Nash's interference was not enough to prevent Hogan from regaining his championship, thanks to Bret Hart's interference shortly thereafter). Nash became the leader of nWo Wolfpac, alongside Savage, Curt Hennig, and Konnan. Hennig, however, shortly thereafter defected over to Hogan's nWo Hollywood faction. Then, during a match between Hall, Nash, Sting and The Giant (who had recently rejoined the nWo after being kicked out two years prior), Hall turned on Nash by hitting him with his tag team title belt and leaving the ring. The Wolfpac, however, was not down for long as Lex Luger joined Nash's team. Sting would eventually become a member as well, after being recruited heavily by both sides towards the middle of 1998. After Sting won Giant's half of the tag team title at the Great American Bash that June, Nash became Sting's partner. They defended the championship until July 20, when they were defeated by Hall and The Giant. Nash then set his sights on his former partner, and the rivalry came to a head at Halloween Havoc on October 25. During the course of the match, Nash jackknifed Hall twice but, instead of pinning him, left the ring and lost via countout.
The following month at World War 3, Nash entered the 60-man, three ring battle royal that was a staple of the pay-per-view, with the winner getting a shot at the WCW World Heavyweight Championship at Starrcade the following month. Nash survived to the end after literally clearing his ring out and big booting Lex Luger, who had Scott Hall in the Torture Rack, over the top rope, and earned his shot at the title. At Starrcade, Nash won the WCW World Heavyweight Championship from Goldberg (who had an officially given 173–0 win-loss record before the match) after Scott Hall shocked Goldberg with a stun gun. In doing so, Nash broke Goldberg's long running undefeated streak. On January 4, 1999, Nash and Goldberg were set to meet in a rematch, but the match did not happen because of Goldberg being arrested for stalking Miss Elizabeth. That night also marked the return of Hulk Hogan after his "retirement" two months prior. With Goldberg unable to wrestle, Nash challenged Hogan instead. Hogan simply poked Nash in the chest, who proceeded to fall down and willingly allow Hogan to pin him for the title. The gesture marked the reunion of the feuding nWo factions into one. The return, however, was short lived, and by the midpoint of 1999, the nWo reunion was over.
In May 1999, Nash won the WCW World Heavyweight Championship for the second time by defeating Diamond Dallas Page at Slamboree. He then appeared on The Tonight Show and put up a $250,000 challenge to Bret Hart for its May 24 program. However, Bret's brother, Owen, died in a wrestling stunt just as Bret was flying to Los Angeles; this immediately canceled their match and feud. Nash then entered a feud with the returning Randy Savage, who was later joined by a returning Sid Vicious at The Great American Bash in June when he powerbombed Nash during the match, thus giving Nash a disqualification victory (as the signature move of both men, the Jackknife Powerbomb, had been ruled an "illegal" move by WCW leadership due to its high potential for injury). This rivalry culminated in a tag team match at Bash at the Beach in July 1999 which pitted Nash and Sting against Savage and Sid. A stipulation was added that whoever got the pin in the match would become WCW World Heavyweight Champion. Nash was pinned by Savage and lost his championship, but would get his revenge the next night on Nitro in a title match between Savage and a returning Hulk Hogan, and in a similar situation to Savage's first title defense from the previous year, he used a Jackknife Powerbomb on Savage, preserving the victory for Hogan. The following week, however, Nash attacked Hogan during a match pitting Hogan against Vicious. Nash, Sid, and Rick Steiner then feuded with Hogan, Sting, and a returning Goldberg until Road Wild, where Hogan defeated Nash in a "retirement" match. On October 4, 1999, Nash returned to WCW along with Scott Hall, which was later revealed to be a new version of the nWo involving Nash, Hall, Bret Hart, and Jeff Jarrett called 'nWo 2000'. This would not last long either due to the injury of Hart, the group's leader, and Nash spent most of 2000 feuding with the likes of Terry Funk, Mike Awesome, Scott Steiner, and Booker T.
Nash won the WCW World Heavyweight Championship again from Booker T on the August 28, 2000 episode of Monday Nitro in Las Cruces, New Mexico. He eventually lost it to Booker T later on at Fall Brawl. He even had a stint as WCW Commissioner, and he served as a coach/mentor to The Natural Born Thrillers, who would eventually turn on Nash. Nash aligned himself with Diamond Dallas Page, reuniting the Vegas Connection, but renamed The Insiders. They feuded with the Perfect Event (Shawn Stasiak and Chuck Palumbo) and won the WCW World Tag Team Championship at Mayhem on November 26, 2000. Shortly after, they were stripped of the title by Commissioner Mike Sanders in mid-December. Weeks later, they won the title back at Starrcade. In 2001 (WCW's final months), the Insiders continued their feud with the Natural Born Thrillers. Nash lost another "retirement" match to Scott Steiner at SuperBrawl Revenge, but it would not be long before WCW announced the sale of the company to the World Wrestling Federation. Nash chose to wait out the remainder of his contract, which expired on December 31, 2001.
Following the expiration of his contract, Nash, along with Scott Hall and Hulk Hogan, were rehired by the WWF. Their rehiring was announced several weeks before their debut, with Vince McMahon claiming to have hired the nWo in order to destroy the WWF – of which Ric Flair was now a co-owner, which McMahon could not tolerate. Billed as the original nWo, Nash, Hall, and 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 claimed to have reformed, 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 episode of Raw, the nWo was officially disbanded by Vince McMahon as Eric Bischoff became Raw General Manager.
After a 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 with Triple H picking up the win for his team, pinning Nash after hitting him with a sledgehammer. Following Backlash, Nash was granted again a shot at Triple H's title, 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 on the August 18 episode of Raw. His last match in WWE was at SummerSlam in an Elimination Chamber match for the World Heavyweight Championship against Triple H, Shawn Michaels, Goldberg, Chris 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.
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 at number 32, but was eliminated by Wade Barrett. It was announced that he had signed a five-year WWE Legends contract. On April 2, Nash, along with X-Pac, was on hand to celebrate the induction of Shawn Michaels into the WWE Hall of Fame class of 2011. Triple H inducted Michaels, and after Michaels gave his speech, Nash and Pac joined the two on stage to celebrate.
Nash, no longer billed as Diesel, returned at SummerSlam in August, attacking CM Punk after he became WWE Champion, which allowed Alberto Del Rio to cash in his Money in the Bank briefcase and win the title, thus turning heel. 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 booked himself in the match against Punk instead, Nash attacked them both at a contract signing and was fired on screen. At Night of Champions, Nash interfered in their match, alongside The Miz and R-Truth. Triple H then attacked Nash with a sledgehammer before winning the match. Nash returned at the following pay-per-view, Vengeance, to help Miz and Truth defeat Punk and Triple H. After the match, he attacked Triple H with a Jackknife Powerbomb and attacked him again the following night with his sledgehammer, preventing him from receiving medical attention and taking him off TV. The following Monday, on October 31, Laurinaitis again signed Nash to a new contract. He continued to appear on Raw, attacking Santino Marella with a Jackknife Powerbomb and cutting promos about how Triple H was more of a boss than a friend. On the December 5 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 TLC: Tables, Ladders and Chairs in a ladder match with a sledgehammer hanging above the ring which he lost by pinfall after a sledgehammer shot to the face, ending the feud in the process.
In late 2012 and early 2013, Nash began appearing on WWE's developmental training show, 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 his finisher, effectively turning face. Nash later again appeared on 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 honorary member of the stable. Nash then helped DX 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. He inducted real-life close friend and former tag team partner, Scott Hall, into the WWE Hall of Fame class of 2014. Nash appeared on the August 11 episode of Raw to reunite the nWo with Hulk Hogan and Scott Hall as part of Hogan's birthday celebration.
On the January 19, 2015 episode of Raw, Nash appeared with X-Pac and Scott Hall to reunite the nWo, and along with The Acolytes Protection Agency and The New Age Outlaws, they beat down The Ascension, who had been insulting legends from past weeks. On March 23, 2015, it was announced that Nash would be inducted into the WWE Hall of Fame class of 2015. On March 28, he was inducted by long-time friend and Kliq member, Shawn Michaels. On March 29, Nash appeared at WrestleMania 31 alongside Hulk Hogan and Scott Hall, in their attempt to even the odds in favor of Sting in his match against Triple H, who had D-Generation X (Billy Gunn, Road Dogg, Shawn Michaels, and X-Pac) in his corner. However, Sting lost the match.
Nash made a return to WWE for the Raw Reunion show on July 22, 2019. On December 9, 2019, it was announced that Nash would be inducted into the WWE Hall of Fame (2020 Class) for a second time as a member of nWo, together with Hogan, Hall, and X-Pac.