On the April 2, 2021 episode of WWE SmackDown, Paul would make his WWE debut, as a guest of Sami Zayn for his red carpet premiere of his documentary, with Zayn later inviting Paul to be at ringside for his match at WrestleMania 37 against Kevin Owens. At the event, after Owens defeated Zayn, Paul would celebrate with Owens, before being hit with a Stunner by Owens. On the September 3 episode of SmackDown, Paul would return to WWE as a special guest of Happy Corbin on The KO Show, where Paul would help Corbin attack Kevin Owens.
On the February 21, 2022 episode of WWE Raw, Paul would be revealed as the tag team partner of The Miz to take on Rey Mysterio and Dominik Mysterio at WrestleMania 38. At the event, Paul and Miz would defeat the Mysterios, however, after the match, Miz would attack Paul.
On June 30, 2022, Paul signed a multi-year contract with WWE. At SummerSlam on July 30, Paul would defeated The Miz in a singles match after putting Miz through the announce table with a Frog Splash.
After Paul appeared on the September 16 episode of SmackDown to challenge Roman Reigns, it was announced during a press conference the following day that Paul would face Reigns at Crown Jewel on November 5 for Reigns' Undisputed WWE Universal Championship. Paul was unsuccessful in defeating Reigns, despite interference from his entourage and his brother Jake, marking his first loss in WWE in what was only his third match. It was later revealed that Paul had suffered a torn meniscus, MCL, and ACL. On January 28, 2023, Paul made his return from injury at the Royal Rumble at number twenty-nine, eliminating Seth "Freakin" Rollins before he was eliminated by the eventual winner Cody Rhodes. At Elimination Chamber on February 18, Paul cost Rollins the United States Championship match inside the Elimination Chamber to Austin Theory. On the March 6 episode of Raw, a match between Paul and Rollins was scheduled for WrestleMania 39. On WrestleMania Night One, Paul lost to Rollins. Paul returned on the June 19 episode of Raw, where he announced that he had been added to the men's Money in the Bank ladder match at the namesake event. Paul would be unsuccessful in winning the match. The next month at SummerSlam, Paul defeated Ricochet after using brass knuckles.
On October 14, Paul called out United States Champion Rey Mysterio. This led to Mysterio appearing on the October 20 episode of SmackDown to accept his challenge, with the match taking place at Crown Jewel. At the event, Paul became the new United States Champion after using brass knuckles to win the match. Paul made his first appearance as champion on the December 1 episode of SmackDown, where he announced a tournament to determine his first challenger. Paul retained the championship via disqualification in a match against Kevin Owens at the Royal Rumble on January 27, 2024. On the February 16 episode of SmackDown, Paul defeated The Miz in his first televised match in WWE to qualify for the men's Elimination Chamber match at Elimination Chamber: Perth. At the event, he was eliminated by Randy Orton before returning later to cost Orton the match. On night two of WrestleMania XL, Paul successfully defended the United States Championship against Orton and Kevin Owens in a triple threat match. On the May 10 episode of SmackDown, General Manager Nick Aldis announced that Paul would be the next opponent against Undisputed WWE Champion Cody Rhodes and a champion versus champion match was scheduled for King and Queen of the Ring. At the event on May 25, Paul failed to win the title from Rhodes after interference from special guest ring announcer Ibrahim Al Hajjaj. At SummerSlam on August 3, Paul lost the United States Championship to LA Knight, ending his reign at 273 days.