In January 2021, Dempsey signed with WWE, where he was assigned to the NXT UK brand. Dempsey made his debut on February 25 edition of NXT UK, where he was defeated by Tyler Bate.
Dempsey later joined Teoman and Rohan Raja to form Die Familie, beginning a feud with Gallus. On the November 18 edition of NXT UK, Die Familie defeated Gallus in their first ever match as a team. Die Familie also started a feud with A-Kid. On the March 10, 2022 edition of NXT UK, Dempsey defeated A-Kid. Matthews was later challenged by A-Kid to a Heritage Rules match. This took place on the April 21 edition of NXT UK, where Dempsey won 2-1 in the sixth round.
Dempsey then competed in a tournament for the NXT United Kingdom Championship, where he lost to Oliver Carter. This ended up being Dempsey’s final match for NXT UK. In August 2022, Dempsey moved to the NXT brand.
On the February 14, 2023 edition of NXT, Dempsey started associating with Drew Gulak, where Gulak served as a teacher for Dempsey. On the April 18 edition of NXT, Dempsey challenged Wes Lee for the NXT North American Championship, which he lost. In the following months, Dempsey and Gulak formed the No Quarter Catch Crew, where they would later be joined by Myles Borne and Damon Kemp.
Throughout August and September, Dempsey competed in the NXT Global Heritage Invitational to crown a number one contender for the NXT Heritage Cup. Dempsey came up empty-handed, getting zero points as he lost to the likes of Axiom (formerly A-Kid), Tyler Bate, and Butch. On the December 13 edition of NXT, Dempsey was involved in a triple threat match for the NXT North American Championship with Joe Coffey and NXT North American Champion Dragon Lee, who retained his title.
On the February 27, 2024 edition of NXT, Dempsey defeated Noam Dar in a Heritage Cup Rules match to become the new NXT Heritage Cup Champion, which is the first championship in his career. At NXT Roadblock, Dempsey and the No Quarter Catch Crew announced that the NXT Heritage Cup would be defended by the whole stable under the "Catch Clause". The Catch Clause was invoked on the March 19 episode of NXT, with Gulak defeating Chase University's Riley Osborne 2–1 to successfully defend the Cup. On the April 16 episode of NXT, The D'Angelo Family leader Tony D'Angelo revealed that his Family were hired by NQCC to eliminate Gulak. D'Angelo demanded payment for the job but NQCC refused, causing a brawl between the two factions and resulting in a six-man tag team match between the two factions during week one of Spring Breakin', which was won by The D'Angelo Family (D'Angelo, Channing "Stacks" Lorenzo and Luca Crusifino). On the May 7 episode of NXT, Dempsey and Borne faced Tyson DuPont and Tyriek Igwe in a tag team match. Before the match started, the match referee was declared "unavailable" and Stacks was announced as the special guest referee, costing Dempsey and Borne the match through a fast count. After the match, an irate Dempsey gave D'Angelo a title shot at the NXT Heritage Cup. One week later, D'Angelo defeated Dempsey 2–1 to win the Cup, ending Dempsey's reign at 76 days. After losing the Cup, Dempsey declared that the NQCC needed structure and a chain of command, and assumed a leadership role. He then ordered Kemp to face D'Angelo in a rematch for the Cup, but Kemp lost the match 0–2 on the June 4 episode of NXT. On the August 13 episode of NXT, Dempsey defeated Tony D’Angelo 2-1 to win the NXT Heritage Cup for the second time after some outside interference from new stablemate Wren Sinclair.
On the December 24 episode of NXT, Lexis King, with Dempsey's father William Regal as his cornerman, faced Dempsey in a rematch for the NXT Heritage Cup. Regal passed his brass knuckles for King to use against Dempsey but King refused. Regal then punched King, to which the brass knuckles fell into the ring. The referee saw Dempsey pick up the brass knuckles and assumed that Dempsey used it on King and disqualified Dempsey. In British Round Rules, the Cup change hands upon disqualification of the reigning champion, resulting in King becoming the new NXT Heritage Cup Champion, ending Dempsey's second reign at 134 days. However, due to a grey area in British Round Rules on whether the Cup changes hand upon disqualification of the reigning champion, NXT general manager Ava ordered King to return the Cup back to Dempsey and announced that they would compete for the Cup in a one-fall match at NXT New Year's Evil on January 7, 2025, where he lost the Cup back to King.