On April 5, 2018, during WrestleMania Axxess weekend, Lee made an appearance for NXT defeating Kassius Ohno. On May 1, it was announced that Lee had signed a contract with WWE. Lee appeared in attendance for NXT TakeOver: Chicago II on June 16.
On the August 8 episode of NXT, Lee made his in-ring debut, defeating Marcel Barthel. He appeared two weeks later on NXT, defeating Luke Menzies. An altercation between Lee and Kona Reeves was shown on the September 26 episode of NXT. The two had a match two weeks later, in which Lee was victorious.
On the November 21 episode of NXT, Lee defeated Fidel Bravo. Later in the night, Lars Sullivan defeated Keita Murray. While Sullivan attacked Murray after the match, Lee intervened and chased Sullivan from the ring. The two faced off the following week, in which Lee was defeated.
On March 7, 2019, Lee suffered an undisclosed injury. The injury resulted in an early end to his planned feud with Dominik Dijakovic, who he was supposed to face for the next two months. Lee returned to action on April 18, at a NXT live event in Lakeland, Florida. He resumed his feud with Dijakovic and lost to him on the August 28, 2019 edition of NXT.
On the November 1 episode of SmackDown, Lee and Matt Riddle were one of the many NXT wrestlers to invade the show, confronting and ultimately attacking Sami Zayn. Later that night, Lee joined Triple H and the rest of the NXT roster as they declared war on both Raw and SmackDown, and vowed to win the Survivor Series brand warfare, but was ultimately pinned by Roman Reigns therefore allowing Team SmackDown to take the victory. On the January 8, 2020 episode of NXT, Lee defeated Cameron Grimes, Damian Priest, and Dominik Dijakovic to become the number one contender to the NXT North American Championship. On the January 22 episode of NXT, Lee defeated Roderick Strong to win the NXT North American Championship, marking his first championship win in WWE and ending the Undisputed Prophecy. Lee participated in the Royal Rumble match and entered at number thirteen at the namesake pay-per-view, but was eliminated by the WWE Champion Brock Lesnar. On the February 5 episode of NXT, it was announced that Lee would defend his North American championship against Dijakovic at NXT TakeOver: Portland, where Lee successfully defended his title. On a following episode of NXT, Lee defeated Cameron Grimes and was attacked. Afterwards, Dominik Dijakovic came into the ring to rescue Lee. Lee, who thought it was a trick, hit Dijakovic with a Spirit Bomb. On the April 1, 2020 episode of NXT, Lee defeated Dijakovic and Priest to retain the championship. In early June, Lee began feuding with Johnny Gargano after they traded shots via video promos involving Gargano's wife Candice LeRae and Lee's girlfriend Mia Yim. Lee and Yim would face the Garganos in a mixed tag team match on June 4, where LeRae picked up the victory for the team. Lee would successfully defend the North American Championship against Johnny Gargano at NXT TakeOver: In Your House. On night two of NXT: The Great American Bash, he defeated Adam Cole to win the NXT Championship, ending Cole's record 396-day reign and becoming a double champion, and the first person to hold both titles simultaneously. He made his first successful defense of both titles against Dominik Dijakovic on the July 16 episode of NXT. On the July 22 episode of NXT, Lee voluntarily relinquished the North American Championship to provide opportunities to other superstars and to solely focus on defending the NXT Championship. On the July 29 episode of NXT, Lee started a feud with Karrion Kross after he made Lee watch as Dominik Dijakovic was being submitted in their match the week before. At NXT TakeOver: XXX on August 22, Lee lost the NXT Championship to Kross in what would be his final match in NXT.
At SummerSlam, the night after Lee's title loss at NXT TakeOver: XXX, it was announced by WWE that Lee would be moving to the Raw brand and make his debut the following day's episode of Raw. On the August 24, 2020 episode of Raw, Lee would make his debut confronting Randy Orton, prompting a one-on-one match between the two. However, the match would end in a disqualification win for Orton following interference from WWE Champion Drew McIntyre. After this, a rematch between Lee and Orton was scheduled for Payback, which Lee would win. On the October 5, 2020 episode of Raw, Lee began a feud with Braun Strowman, facing Strowman in an exhibition match which ended in disqualification after interference from Retribution. Lee would remain on the Raw brand during the 2020 WWE Draft, and went on to face Strowman yet again on the October 19, 2020 edition of Raw, which Lee lost. After the match, Lee attacked Strowman. Lee would then qualify for Team Raw at Survivor Series after beating Elias. At Survivor Series, Lee would eliminate Jey Uso and get Team Raw the victory. The following night on Raw, Lee would beat Bobby Lashley via disqualification to advance to a triple threat, where the winner would face Drew McIntyre for the WWE Title at TLC. On the November 30 episode of Raw, Lee failed to win after AJ Styles pinned Riddle to win the match.
On the December 28 edition of Raw, Lee defeated Sheamus to earn a WWE Championship match against Drew McIntyre. On the main event of the January 4, 2021 edition of Raw, he was unsuccessful in winning the championship. The following month, Lee was scheduled to face Riddle and Bobby Lashley for the United States Championship at the Elimination Chamber event, but was pulled from the event due to an injury. After a five month absence, Lee returned on the July 19 episode of Raw, accepting WWE Champion Bobby Lashley's open challenge, but was defeated. On the July 26 episode of Raw, he lost a match to NXT Champion Karrion Kross, whom he would defeat the next week.
On the September 27 episode of Raw, Lee, now known as Keith "Bearcat" Lee, defeated Akira Tozawa in less than a minute. On November 4, Lee was released from his WWE contract.