As The Lightning Kid, X-Pac had his WWF tryout match in 1993 in Phoenix, Arizona, the day after WrestleMania IX, against fellow hopeful Louie Spicolli.
After earning a contract, he made his television debut as The Kamikaze Kid on Monday Night Raw on May 3, losing to Doink the Clown. He quickly became The Cannonball Kid, then simply The Kid. He scored an upset pinfall on Razor Ramon on the May 17 episode of Monday Night Raw, thus becoming The 1–2–3 Kid. Razor challenged him to a rematch, wagering $10,000 of his own money. Kid accepted the challenge, but grabbed the money and ran from the arena during the match. Ted DiBiase, who was feuding with Razor, taunted him over losing to a nobody. This angered Kid, and led to a match in which he upset DiBiase as well. Razor turned face shortly after and took Kid under his wing. The 1–2–3 Kid made his pay-per-view debut at SummerSlam, losing to DiBiase's tag partner Irwin R. Schyster after Razor had defeated DiBiase. At Survivor Series, Kid was on Razor's team in a four-on-four elimination match. He and Marty Jannetty were the sole survivors, which led to them forming a tag team and holding the WWF Tag Team Championship for a week in January 1994 after beating The Quebecers.
For the next two years, The 1–2–3 Kid was a natural underdog and fan favorite. He wrestled Bret Hart in an unusually long (for the time) and highly acclaimed match for the WWF World Heavyweight Championship on July 11, 1994 on Raw and had another brief (one-day) tag title reign in January 1995, with Bob Holly, beating Bam Bam Bigelow and Tatanka at the 1995 Royal Rumble before losing to The Smoking Gunns on Raw.
After Kid and Razor failed to win back the Tag Team Championship from Billy Gunn and Bart Gunn in October 1995, Kid attacked the face Gunns after the match to tease a heel turn. On the Raw before the 1995 Survivor Series in November, he was the guest referee in a match between Razor Ramon and Sycho Sid. As Razor attempted his finisher, The Razor's Edge, Kid pulled Sid down from Razor, allowing Sid to then hit Razor with his Powerbomb, and Kid fast-counted the pinfall, thus turning heel. He was the sole survivor of his Survivor Series match, besting rival Marty Jannetty. Razor and Marty would team up to beat Kid and Sid at In Your House 5.
After Survivor Series, Kid joined Ted DiBiase's Million Dollar Corporation faction. He remained with the group until May 1996 when he left the WWF. He lost a "Crybaby match" to Razor Ramon at In Your House 6. The 1–2–3 Kid's final WWF match aired on the May 20 episode of Monday Night Raw; he lost to Savio Vega.
On September 16, 1996, Kid was shown sitting in the front row for a live episode of WCW Nitro. Later that night, he stood and used a remote control to release New World Order (nWo) propaganda from the ceiling, revealing himself as the newest member of the recently formed faction. He was called Syxx, because he was the sixth member of the nWo, and six is the sum of numbers in "1–2–3 Kid".
In his first major angle, Syxx stole Eddie Guerrero's WCW United States Heavyweight Championship belt, leading to a ladder match for the title at Souled Out in January 1997, which Syxx lost. The next month, at SuperBrawl VII, Syxx pinned Dean Malenko for the WCW Cruiserweight Championship, after hitting him with the title belt, which he had grabbed from Guerrero at ringside. In June 1997, he lost the championship to Chris Jericho at a webcast house show in Los Angeles, California, minutes after successfully defending against Rey Mysterio, Jr.
During a feud with Ric Flair, and a loss to him at Road Wild in August, Syxx disparagingly portrayed Flair as part of an nWo segment parodying his Four Horsemen group. This segment led to a WarGames match at Fall Brawl, where Syxx, Kevin Nash, Buff Bagwell and Konnan defeated The Four Horsemen (Flair, Steve McMichael, Chris Benoit and Curt Hennig) after Hennig betrayed the Horsemen and joined the nWo. In mid-1997, the nWo invoked "Wolfpac Rules", allowing Syxx to replace the injured Kevin Nash in defending the WCW World Tag Team Championship with Scott Hall. On October 13, 1997, Hall and Syxx lost the title to The Steiner Brothers (Rick and Scott).
In October, a neck injury sidelined Syxx from wrestling, but he continued to appear on television for several weeks after. While injured, he was released from WCW.
Syxx returned to WWF television on the March 30, 1998 episode of Monday Night Raw, the night after WrestleMania XIV and days after his firing from WCW. With Shawn Michaels beginning a four-year retirement after a WWF World Heavyweight Championship loss and back injury, Triple H was now the leader of D-Generation X (DX). He said 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." Syxx appeared on the stage, commented on Eric Bischoff and Hulk Hogan, and said if they weren't contracted to WCW, Hall and Nash would have also returned to the WWF. Bischoff responded on Nitro the next week by telling Syxx to "bite me".
Initially called "The Kid" on the WWF website, he became known as X-Pac by the next Raw. X-Pac feuded with Jeff Jarrett, ultimately defeating him in a hair-vs-hair match at SummerSlam, then with WWF European Champion D'Lo Brown, whom he dethroned on September 21, 1998. He lost the title to Brown two weeks later, then won it again at Judgment Day: In Your House in October. X-Pac lost the title to Shane McMahon on February 15, 1999. At WrestleMania XV, he lost a championship rematch when Triple H betrayed him, and hit him with his Pedigree finisher.
X-Pac then sided with Road Dogg against Triple H, Chyna and Billy Gunn, after the temporary demise of DX. X-Pac and Road Dogg wanted a reformed DX to be about rebellion, while the others wanted it to be about making money. X-Pac befriended Kane, a mute, angry loner whom he partially socialized and encouraged to speak, through an electrolarynx. They won the WWF Tag Team Championship twice together. After DX reunited in late 1999 as a heel group, X-Pac led Kane to believe he would be inducted into DX, but instead betrayed him and eventually stole his new girlfriend, Tori.
In 2000, X-Pac feuded with fellow DX member Road Dogg and Chris Jericho, whose botched powerbomb sidelined X-Pac for three months with another neck injury.
After returning from his neck injury in early 2001, X-Pac briefly feuded with Chris Jericho over the WWF Intercontinental Championship, culminating in a fatal four-way match with Chris Benoit and Eddie Guerrero at No Way Out in February, which Jericho won.
In February, X-Pac formed a new stable called X-Factor with Justin Credible and Albert. During this time, he usually teamed with Credible, but also appeared in singles matches. He won the WWF Light Heavyweight Championship from Jeff Hardy on June 25, then won the WCW Cruiserweight Championship for the second time in his career when he defeated Billy Kidman on July 30. Despite this occurring during the WCW Invasion, where the WWF side was mostly portrayed as the good guys, the fans were vocal in their disapproval of X-Pac during his reign. This was acknowledged on-screen by both Alliance member Billy Kidman and, later by WWF member Edge.
When Credible joined the ECW/WCW Alliance, X-Factor broke up. At SummerSlam 2001, X-Pac defeated Tajiri to win the WWF Light Heavyweight Championship for the second time and was WCW Cruiserweight Champion at the same time. X-Pac then feuded with Kidman and Tajiri. After losing the WCW Cruiserweight title to Kidman, he took time off for another injury. The WWF Light Heavyweight Championship was abandoned upon his return to television, though he defended it at several house shows shortly prior.
Scott Hall, Kevin Nash and Hollywood Hogan returned to the WWF in 2002 as the New World Order, brought in by Vince McMahon. Hogan was kicked from the group after losing to The Rock at WrestleMania. X-Pac, who had been out with an injury, returned on the March 21 episode of SmackDown!, rejoined the nWo and attacked Hogan. The storyline was dropped after the first WWE Draft, when the nWo went to Raw and Hogan to SmackDown.
During the nWo's feud with Booker T and Goldust, a botched spinebuster injured him and kept him from participating in the feud. On the July 8, 2002 episode of Raw, X-Pac wrestled in his last WWE match, a ten-man tag. This match was also the end of the nWo angle, as Nash tore his quadriceps, and Vince McMahon disbanded the group a week later.
At SummerSlam, Raw commentator Jim Ross announced WWE and X-Pac had parted ways.
On April 2, 2011, X-Pac returned to WWE television to celebrate, with Kevin Nash and Triple H, their longtime friend Shawn Michaels' induction into the 2011 WWE Hall of Fame.
In 2012, he attended the Hall of Fame again, with The Kliq. On July 23, he, Billy Gunn and Road Dogg returned to join Shawn Michaels and Triple H for a D-Generation X reunion on the 1000th episode of Raw. In early 2013, The Kliq/DX reunited for an episode of NXT. In early March 2013, he signed a WWE Legends contract.
On April 5, 2014, the night before WrestleMania XXX, X-Pac joined Shawn Michaels, Triple H, and Kevin Nash at Scott Hall's WWE Hall of Fame induction, reuniting The Kliq.
On the January 19, 2015 episode of Raw, labelled Raw Reunion, Damien Mizdow 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 Mizdow 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. At WrestleMania 31, X-Pac along with the New Age Outlaws and Shawn Michaels helped Triple H win his match against Sting. X-Pac returned at Raw 25 and reunited with DX and Scott Hall. He attended NXT TakeOver: WarGames in 2018.
In 2019, X-Pac was introduced into the WWE Hall of Fame as a member of D-Generation X alongside Triple H, Shawn Michaels, Road Dogg, Billy Gunn and the late Chyna.
On December 9, 2019, it was announced that X-Pac (under his real name this time) would be inducted into the WWE Hall of Fame a second time as a part of the class of 2020, this time as a member of the New World Order, together with fellow former nWo stablemates Hulk Hogan, Kevin Nash, and Scott Hall; this made him the first person to be inducted two years in a row, and the first to be inducted twice as parts of teams or groups.