Sean Waltman (X-Pac) signed a contract in 1993 with the company, debuting as the Lightning Kid and losing his debut match to Rad Radford. After several weeks as a jobber, he upset Razor Ramon on the May 17, 1993 episode of Monday Night Raw. Waltman was given a number of different names such as the Kamikaze Kid, Cannonball Kid, and The Kid. Waltman was then renamed the 1–2–3 Kid. Waltman was worked into Ramon's feud with Ted DiBiase. DiBiase taunted Ramon repeatedly over losing to a nobody until he too was pinned by the Kid. Ramon turned face shortly after and took The Kid under his wing.
For the next two years, Waltman's character played the role of the natural underdog and displayed a rebellious attitude until the Raw before the 1995 Survivor Series pay-per-view where he was the guest referee in a match between Razor Ramon and Sycho Sid. As Ramon was attempting his finisher, the Razor's Edge, The Kid pulled Sid down and fast counted him, giving Sid the win and thus turning heel. After this event, the Kid joined Ted DiBiase's Million Dollar Corporation faction. The 1–2–3 Kid remained with this group until the end of Waltman's first run in the company. In his first run, he held two WWF Tag Team titles, once each with Marty Jannetty and Bob Holly, and came close to defeating Bret Hart for the WWF Championship in July 1994.
Waltman returned to WWF programming on March 30, 1998, the night after WrestleMania XIV. With Shawn Michaels out of action following his title loss and back injury, which kept him inactive for four years, 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.
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 as a villainous group, 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.
Upon his return to action, Waltman briefly feuded with Chris Jericho over the WWF Intercontinental Championship culminating in a Fatal 4 Way match won by Jericho at No Way Out 2001 which also featured Chris Benoit and Eddie Guerrero. Waltman's 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 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 Alliance, he feuded with Kidman and Tajiri, until he took time off for another injury, after losing the 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 on the March 21 episode of SmackDown! and immediately attacked Hogan. 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.
From there they would start a feud with Booker T and Goldust. On August 25, 2002, Raw commentator Jim Ross announced that WWE and Waltman had parted ways
On April 2, 2011, Sean Waltman made his return to WWE television to celebrate his longtime friend Shawn Michaels induction to the 2011 WWE Hall of Fame alongside his Kliq buddies Kevin Nash and Triple H. Since then, he has been scouting and evaluating talent in NXT Wrestling (then Florida Championship Wrestling), the WWE developmental territory. In 2012, Waltman attended the Hall of Fame again with the Kliq. On July 23, 2012, he, alongside Billy Gunn and Road Dogg, returned to join Shawn Michaels and Triple H for a D-Generation X reunion at the 1000th episode of Raw. In early 2013, The Kliq/DX reunited for an episode of WWE NXT.
On April 5, 2014, the night before WrestleMania XXX, Waltman joined Shawn Michaels, Triple H, and Kevin Nash at Scott Hall's WWE Hall of Fame induction, reuniting The Kliq.
On January 19 edition of Raw, labelled Raw Reunion, Damien Sandow appeared with Kevin Nash, Shawn Michaels, Scott Hall and Triple H dressed as X-Pac, only for X-Pac to confront him (finding the impersonation funny), until The Miz interrupted them to tell them that Sandow was only his stunt double, and that the party was over. He appeared again with Hall and Nash, being interrupted by The Ascension, who were attempting to attack them, only to be joined by The APA and The New Age Outlaws in fending off The Ascension.