On the April 5, 2004 episode of Raw, Eugene made his WWE debut. He was portrayed as the "special" and overly excited nephew of Raw General Manager Eric Bischoff. Bischoff soon placed him under the care of the newly returned William Regal. In May 2004, Eugene began a feud with Jonathan Coachman, who attempted to banish him from WWE (only to be stopped by The Rock) culminated in Eugene's pay-per-view debut, when he defeated Coachman at Bad Blood. Eugene then feuded with Triple H, culminating in a match at SummerSlam, which Eugene lost.
Allied with William Regal, he then feuded with his uncle, Eric Bischoff. This led to a match at the first-ever Taboo Tuesday pay-per-view on October 19. The fan-chosen (via voting at WWE's website) stipulation for the match was that the loser would have his head shaved. Eugene won this match (with help from Mr. McMahon) and subsequently shaved Bischoff's head.
With Regal, Eugene won his first and only championship in WWE, taking the World Tag Team Championship from La Résistance on the November 15 episode of Raw. Eugene (legitimately) injured himself at New Year's Revolution, in a match with Christian and Tomko, after botching a dropkick. He suffered a ruptured left patellar ligament, which required surgery and sidelined him for several months. This also cost him the World Tag Team Championship, which Regal lost when forced to defend, teamed with Jonathan Coachman as a substitute.
During recovery, Eugene made a surprise appearance at WrestleMania 21, delivering a promo until being interrupted and attacked by villains Muhammed Hassan and Daivari. This set up the WWE return of Hulk Hogan, who came to the ring to save Eugene from the beating.
Eugene returned to wrestling on the July 24, 2005 episode of Raw, taking part in Kurt Angle's regular "Kurt Angle Invitational" challenge. Eugene defeated Angle, winning his cherished Olympic gold medal (per match stipulation), and starting a feud between the two. The feud culminated at SummerSlam, where Angle defeated Eugene to regain his medal. Eugene then briefly teamed with Tajiri (who, incidentally, had also previously teamed with William Regal). The team lasted until Tajiri's release in December 2005.
At Taboo Tuesday, Rob Conway teamed with Tyson Tomko to face Eugene and a "WWE Legend", to be chosen by the fans. "Superfly" Jimmy Snuka was chosen over Kamala and "Hacksaw" Jim Duggan, and pinned Conway with his Superfly Splash to win the match.
Eugene made his return to WWE in the Royal Rumble match on January 29, 2006, as the eighteenth entrant. He was eliminated by Chris Benoit. Eugene briefly feuded with Matt Striker, after appearing in the "Matt Striker's Classroom" segment of Backlash on April 30, 2006. Striker ultimately defeated Eugene to end the feud. After appearing at One Night Stand, Eugene formed a tag team with "Hacksaw" Jim Duggan. He challenged Umaga to a match at Vengeance and lost, despite having Duggan, Doink the Clown, and Kamala in his corner. He defeated Vince McMahon and Shane McMahon in a handicap match on the July 10 RAW,after interference from D-Generation X. On the September 3 Raw, Eugene and Duggan challenged World Tag Team Champions the Spirit Squad for the title, but were unsuccessful. The team split up on the November 6 Raw. After losing a rematch to The Spirit Squad, Eugene attacked Duggan, turning heel for the first time in his WWE career. However, the angle was soon dropped and, before the end of the year, Eugene became a face again. After this, he wrestled mainly on the RAW B-show, Heat.
On the March 19, 2007 episode of Raw, Eugene was forced into a match with Umaga, as a punishment for accidentally spilling a drink on Vince McMahon. After losing, McMahon shaved Eugene's head. Eugene wrestled on the June 2, 2007 episode of Saturday Night's Main Event, where he debuted a new "Superman-esque" wrestling attire. He teamed with Kane and Doink The Clown to defeat Kevin Thorn, Viscera, and Umaga.
On June 17, Eugene was drafted to SmackDown as their final pick of the 2007 Supplemental Draft. On the June 29 episode of SmackDown!, Eugene debuted for the brand, losing to The Great Khali. The following week, he was tricked into thinking Deuce 'n Domino were helping him win his match against Mark Henry, which led to a quick defeat.
Eugene participated in a battle royale for the World Heavyweight Championship, and was eliminated by Batista and Kane. Eugene made his final WWE appearance on the August 31 edition of SmackDown!. While shooting T-shirts into the crowd via t-shirt gun, he accidentally shot Mark Henry. Henry retaliated by squeezing him with a bearhug until he passed out. On September 1, 2007, Eugene was released from his WWE contract after failing his second drug test.
In July 2009, Eugene signed a contract to return to WWE. On August 1, Eugene defeated Ricky Ortiz at a SmackDown/ECW house show in Manchester, New Hampshire. On the August 10 Raw, Eugene returned to television, losing a Contract on a Pole match to The Calgary Kid, who afterwards removed his mask to reveal himself as The Miz. Four days later, Eugene was once again released from WWE, as a wrestler and a trainer.
Eugene was re-hired by WWE on September 20, 2013, as a trainer of NXT, and was released on October 2, 2014.