In February 1997 at SuperBrawl VII, Syxx had another opportunity for championship gold, defeating Dean Malenko to officially become the Cruiserweight Champion. In June 1997, he lost the Cruiserweight Title to Chris Jericho at a non-televised webcast house show in Los Angeles, California minutes after a successful defense against Rey Mysterio, Jr. Syxx then began a feud with Ric Flair, losing to Flair at Road Wild in August. Syxx was then involved in a parody of the Four Horsemen stable, where he portrayed Ric Flair. This led to the WarGames match at Fall Brawl, with Syxx teaming with Kevin Nash, Buff Bagwell, and Konnan to face the Four Horseman team of Flair, Steve McMichael, Chris Benoit, and Curt Hennig. The nWo won the match after Hennig betrayed the Horsemen and joined the nWo.
Syxx briefly substituted for Kevin Nash, as part of the Outsiders with Scott Hall, in defending the WCW tag-team titles. On October 13, 1997, Hall and Syxx lost the titles to the Steiner Brothers (Rick and Scott).
During that October, Waltman injured his neck and was forced out of action. While he was out injured, he was fired by then WCW President Eric Bischoff as a show of force to keep Hall and Nash in check due to locker room instability caused by the three.
Waltman returned to WWF programming on March 30, 1998, the night after WrestleMania XIV and days after being released from WCW. With Shawn Michaels out of action following his title loss and back injury, Triple H was now the leader of D-Generation X (DX). He stated that he was forming a DX army and "when you start an army, you look to your blood ... you look to your buddies ... you look to your friends ... you look to The Kliq." Waltman appeared on the stage and commented on Bischoff and Hogan, as well as claiming that if contractually free to do so, Hall and Nash would have been joining him in returning to the WWF. This comment actually prompted Bischoff to respond on Nitro the following week, telling Waltman to "bite me".
Following that Raw episode, Waltman was initially being billed as "The Kid" on the WWF homepage, but became known as "X-Pac" by the next broadcast. As a member of DX, Waltman feuded with many wrestlers such as Jeff Jarrett, and he eventually won the WWF European Championship from D'Lo Brown in 1998. He and Brown traded the title a couple of times with Waltman ultimately winning the championship at Judgment Day: In Your House in October 1998. Waltman held the European Championship until February 1999 when he lost the title to Shane McMahon. At WrestleMania XV in 1999, he had a rematch with McMahon for the European Championship, but Triple H betrayed him and cost Waltman the title.
Waltman then paired off with Road Dogg against Triple H, Chyna and Billy Gunn, after the temporary demise of DX. Waltman and Road Dogg wanted DX to be about rebellion while Triple H, Chyna and Billy Gunn wanted it to be about making money. A few weeks after WrestleMania, he became a fan favorite and formed an alliance with Kane; a mute, brooding, menacing loner. Waltman and Kane went on to hold the WWF Tag Team Championship two times. After DX was reunited in late 1999, Waltman led Kane to believe that he would be inducted into DX, but instead betrayed him and eventually stole his girlfriend, Tori. In 2000 he feuded with fellow DX member Road Dogg and Chris Jericho. Waltman was out of action for three months due to a neck injury caused when Jericho botched a powerbomb later in the same year.
Upon his return to action, Waltman found himself without an angle. His most notable actions in the next two years were forming the short-lived faction X-Factor along with Justin Credible and Albert. During this time, he won the WWF Light Heavyweight Championship and the WCW Cruiserweight Championship, held by Billy Kidman, and became the first and only person to hold both belts simultaneously. When X-Factor broke up due to Credible joining The WCW/ECW Alliance, he feuded with Kidman and Tajiri, until he had to take time off for another injury, after losing the WCW Cruiserweight Title to Kidman. The WWF Light Heavyweight Championship was abandoned upon his return to WWF television, although he defended the belt on several house shows just before his return to television.
When Hall, Nash and Hogan returned to the WWF in 2002, Hogan was removed from the latest incarnation of the nWo. Waltman, who had been out with an injury, returned and immediately attacked Hogan, claiming he had been waiting four years to do so, stemming from shoot comments from Hogan on WCW Thunder in 1998 after he was fired that he could not "cut the mustard." This storyline was immediately dropped with the nWo members being drafted to Raw while Hogan being drafted to SmackDown! in the first ever WWE Draft Lottery.
During the nWo's feud with the tag team of Booker T and Goldust. Waltman was often misrepresented as refusing to lose to Booker T, but he denied this in an interview in November 2006, referencing a botched spine buster giving him an injury and keeping him from participating. On August 25, 2002 Raw commentator Jim Ross announced that the WWE and Waltman had parted ways.