Managed by Mr. Fuji, Neidhart debuted for the World Wrestling Federation (WWF) on January 21, 1985, in Madison Square Garden, defeating Tony Garea. He wrestled Bret Hart, who had a cowboy gimmick, to a draw twice. Bret, upset with his gimmick, suggested to Vince McMahon that he form a tag team with Neidhart.
The new team was dubbed The Hart Foundation and was managed by Jimmy Hart (no relation to Bret). They made their pay-per-view debut at WrestleMania 2, where they were the last two eliminated from a 20-man battle royal by André the Giant. The Hart Foundation won their first WWF World Tag Team Championship on the February 7, 1987 episode of WWF Superstars (taped January 26) from The British Bulldogs (Davey Boy Smith and The Dynamite Kid), with the help of referee Danny Davis, who was continually "distracted" by checking on Dynamite (laid out of the match early by a megaphone shot from Jimmy), allowing the challengers to double-team Smith. Davis was subsequently fired as referee and began wrestling, aligned with The Hart Foundation. They lost the titles on the November 7 episode of Prime Time Wrestling (taped October 27), to Strike Force (Rick Martel and Tito Santana).
The Hart Foundation wrestled in another 20-man battle royal at WrestleMania IV. Bad News Brown attacked and eliminated Hart to win. Neidhart eventually joined Bret's side in the feud with Brown causing a rift between the team and manager Jimmy Hart. This led them to fire him and before starting a feud with The Fabulous Rougeau Brothers. In the fall of 1988, Jimmy Hart began managing the Rougeaus. The Hart Foundation unsuccessfully challenged Demolition (Ax and Smash) for the tag title at the 1988 SummerSlam, when Ax hit Hart with Jimmy Hart's megaphone for the pin (Hart accompanied Demolition's manager, Mr. Fuji, solely to further his feud with the challengers). The Hart Foundation continued feuding with Jimmy Hart's wrestlers for the next year, teaming with Jim Duggan to defeat Dino Bravo and The Rougeaus in a 2/3 falls match at the 1989 Royal Rumble, then defeating The Honky Tonk Man and Greg Valentine at WrestleMania V.
In April 1989, both Hart Foundation members began wrestling singles matches at house shows. Neidhart also had a few shots at "Ravishing" Rick Rude's Intercontinental Heavyweight Championship in August.
The Hart Foundation reformed full-time on March 24 in Las Vegas. At WrestleMania VI in Toronto, they defeated The Bolsheviks (Nikolai Volkoff and Boris Zhukov) in 19 seconds. They started a second feud with champions Demolition, who at this point added Crush. At SummerSlam on August 27, The Hart Foundation won the title for a second time in a 2/3 falls match.
On October 30, 1990, The Rockers (Shawn Michaels and Marty Jannetty) defeated The Hart Foundation in a two out of three falls match in Fort Wayne, Indiana, to seemingly win the titles. During the match, the top rope broke by accident, and the match did not air on television. The Rockers defended the WWF Tag Team title against Power and Glory (Paul Roma and Hercules) on November 3, 1990. Shortly after November 3, the WWF decided to not air the title change and that the title would revert to the Hart Foundation. The WWF has never officially recognized The Rockers' reign.
The Hart Foundation reigniting their feud with Jimmy Hart via his team, Rhythm and Blues (Honky Tonk Man and Greg Valentine). Jimmy Hart once again cost The Hart Foundation the tag titles at WrestleMania VII, in a match with his new team, The Nasty Boys (Brian Knobbs and Jerry Sags), when he distracted the referee, allowing Sags to knock out Neidhart with his motorcycle helmet. Hart and Neidhart again split up, though they reunited for a title rematch with The Nasty Boys on the July 29 Prime Time Wrestling, losing by disqualification when Bret hit both champions with a helmet, again introduced by Jimmy Hart. Neidhart also commentated on Wrestling Challenge alongside Gorilla Monsoon and Bobby Heenan from March until August 1991.
Neidhart returned to TV action on the November 9 edition of Superstars of Wrestling against Ric Flair, who continued to apply his figure-four leglock after winning the match, beginning a series of Flair versus Neidhart matches that have continued into a second generation. While hobbling away to the back, he was attacked and further injured by The Beverly Brothers. He was thus replaced in the 1991 Survivor Series by Sgt. Slaughter (who became the team captain).
Neidhart returned on the December 1 edition of Wrestling Challenge, teaming with Owen Hart (Bret's younger brother) as The New Foundation, defeating Barry Horowitz and Duane Gill. Clad in bright parachute pants with checkerboard designs, The New Foundation's highlight was a win over The Orient Express (Kato and Tanaka) at the 1992 Royal Rumble. Aside from one match against The Barbarian and Warlord, they wrestled The Beverly Brothers exclusively at house shows in 1992. He was released on February 16, 1992.
He debuted for World Championship Wrestling (WCW) on the May 15, 1993 episode of WCW WorldWide, beating a jobber. After defeating another jobber on the next episode, Neidhart teamed with The Junkyard Dog for the next, again beating jobbers. Neidhart and The Junkyard Dog beat Paul Orndorff and Dick Slater by disqualification on the June 5 WCW Saturday Night. Eleven days later, he beat Shanghai Pierce in a dark match before Clash of the Champions XXIII. After losing to Maxx Payne at a house show in Kokomo, Indiana, on October 7, Neidhart left WCW.
On November 13, 1993, Neidhart wrestled The Sandman to a no contest at ECW's November to Remember in the ECW Arena.
Neidhart returned to the WWF at the King of the Ring in 1994, as Bret Hart's cornerman for his WWF World Heavyweight Championship match defense against Intercontinental Champion Diesel. After Diesel hit Hart with his Jackknife finisher, Neidhart interfered to prevent the pin, and disqualifying Hart but allowing him to retain the title. After the match, Diesel and Shawn Michaels beat down Hart, and Neidhart didn't intervene. Later that night, Neidhart reappeared at ringside again during Owen Hart's King of the Ring tournament final against Razor Ramon. He attacked Razor outside the ring, behind the referee's back, before Neidhart throwing Razor back for Hart to elbow drop and pin to become "The King of Harts", turning heel for the first time since 1988.
Owen had been feuding with Bret since the Royal Rumble. Neidhart, believing Bret had held Owen back from his potential, sided with Owen, and usually cornered him in matches through the summer. Neidhart claimed he'd only helped Bret keep the WWF World Heavyweight Championship at King of the Ring so Owen could take it from him. This opportunity came in a steel cage match at SummerSlam. Neidhart sat in the third row during the match, behind other Hart family members. After Bret won the match, Neidhart entered the cage, locked it and helped Owen beat him down, while the Hart family members tried to climb over it and save him.
Neidhart joined Owen on Shawn Michaels' team, The Teamsters, to face Razor Ramon and The Bad Guys in an elimination match at Survivor Series. After they eliminated every Bad Guy except Razor, Michaels inadvertently hit Diesel with the Sweet Chin Music. This caused an argument and the tag team partners split up, before Diesel chased Michaels down the aisle. As the other Teamsters tried to intervene, all were counted out.
Because Diesel and Michaels were WWF Tag Team Champions when they split, the titles were vacated and a tournament held. Neidhart and Hart lost to The Headshrinkers (Fatu and Sionne) in the first round on the December 31 Superstars, by disqualification. By the time it aired, Neidhart had left the WWF. Owen wound up winning the WWF Tag Team Championships at WrestleMania XI with new partner Yokozuna and years later with fellow brother-in-law The British Bulldog.
He then returned to ECW where, on April 28, 1995, he lost to Marty Jannetty. The next night he lost to Ron Simmons.
On the July 6, 1996 Superstars, he returned as the masked heel Who, a gimmick designed for commentators Vince McMahon and Jerry Lawler to make "Who's on First?"-style jokes during his matches (most of which he lost). Who last appeared on TV in the "Bikini Beach Blast-Off" party on the SummerSlam pre-show. His last match was a win over Alex Porteau in Miami on September 12.
Neidhart returned to the WWF on the April 28, 1997 episode of Raw is War, attacking Stone Cold Steve Austin and reuniting with Bret and Owen Hart as part of the new Hart Foundation, a stable of Canadian sympathizers, also including Davey Boy Smith and Brian Pillman. On July 6 at In Your House 16: Canadian Stampede in Bret's hometown Calgary, The Hart Foundation defeated the American team of Stone Cold Steve Austin, Ken Shamrock, Goldust and The Legion of Doom. Neidhart was part of Team Canada at Survivor Series in Montreal, teaming with The British Bulldog, Doug Furnas and Philip Lafon to defeat Team USA (Vader, Goldust, "Marvelous" Marc Mero and the debut of "The Lethal Weapon" Steve Blackman) (Bulldog was the sole survivor, Neidhart was pinned by Vader).
After Bret and Davey Boy Smith left the WWF on bad terms because of the Montreal Screwjob at Survivor Series, D-Generation X leader and WWF Champion Shawn Michaels offered Neidhart a spot in the group on the November 24 episode of Raw is War. Neidhart accepted, only for it to be revealed as a setup, as the group assaulted Neidhart at the end of the show. The following week on the December 1 episode of Raw is War, DX member Triple H defeated Neidhart. After the match, DX attacked Neidhart once again and spray-painted "WCW" on his back, and signifying him following Bret Hart to World Championship Wrestling. Sgt. Slaughter and Ken Shamrock saved him, before Slaughter and Shamrock attacked D-Generation X at the end of the show. After Raw is War, WWF announced that Neidhart was released from his WWF contract on December 2, 1997.
In January 1998, Neidhart returned to World Championship Wrestling, where he formed a short-lived tag team with The British Bulldog. They were rarely utilized by WCW and achieved little in-ring success. His final match for WCW was on the September 26 episode of WCW Saturday Night, where Neidhart and The British Bulldog lost to Stevie Ray and Vincent. Neidhart was released from the company in October.
On Raw XV, the 15th-anniversary WWE Raw special on December 10, 2007, Neidhart returned to WWE for the first time since 1997, and participated in the 15th Anniversary Battle Royal, eventually making it to the final five before being eliminated by Skinner.
Neidhart made appearances on the WWE and E! reality show Total Divas, on which his daughter Natalya was a main cast member.
On August 13, 2018, Neidhart passed away.
In 2019, Neidhart (along with Bret Hart) was posthumously inducted into the WWE Hall of Fame as part of the Hart Foundation.