Eugene, as Nick Dinsmore, appeared in World Championship Wrestling (WCW) from August 1998 to January 1999, mainly used as a jobber on WCW Saturday Night. He also worked on two episodes of Thunder (losing to Ernest Miller and Kanyon), and on two episodes of Nitro (first teaming with Lenny Lane to lose a handicap match to Scott Steiner, then losing a singles match to Wrath).
Dinsmore also made some appearances as a jobber in the World Wrestling Federation (WWF), first on an episode of Shotgun Saturday Night in 1999.
On the April 5, 2004 episode of Raw, Dinsmore made his World Wrestling Entertainment (WWE) debut as Eugene. He was portrayed as the "special" and overly excited nephew of Raw General Manager Eric Bischoff, who had savant-like wrestling ability from watching his favorites on television. Bischoff soon placed him under the care of the newly returned William Regal. Although Regal was originally dispirited at having to manage Eugene, he would grow fond of him and the two would quickly become firm friends. In May 2004, Eugene began a feud with Jonathan Coachman, who attempted to banish him from WWE (only to be stopped by The Rock). The feud culminated in Eugene's pay-per-view debut, when he defeated Coachman at Bad Blood. Eugene then feuded with Triple H, which led to match at SummerSlam, which Eugene lost. Two weeks later on Raw, they fought a no disqualification match, which Eugene won after interference from Randy Orton. The next week, they had a steel cage match, which Eugene lost and suffered a kayfabe dislocated shoulder, ending the feud.
After taking a few weeks off to sell his injuries, he 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 suffered an injury at New Year's Revolution on January 9, 2005, in a match with Christian and Tyson Tomko, after botching a dropkick. Although they won the match, he suffered a ruptured left patellar ligament, which required surgery and sidelined him for six months. This also cost him the World Tag Team Championship, which Regal lost to La Résistance when forced to defend at a house show on January 16, 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 Muhammad Hassan and Daivari. He would be saved by the returning Hulk Hogan, after he attacked the villainous duo.
Eugene returned to wrestling on the July 25, 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.
On November 18, 2005, WWE.com reported Eugene was suspended for thirty days for violating their new Wellness Policy.
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. At WrestleMania 22, Eugene competed in a pre-show battle royal which was won by Viscera. 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 4 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 Saturday Night's Main Event XXXIV, 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 royal 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.
In July 2009, Eugene signed a contract to return to WWE. On August 1, he 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. He was re-hired by WWE on September 20, 2013, as trainer for NXT, where he lasted a year before he was released on October 2, 2014.