On March 14, 2021, it was reported that Waller had signed with WWE. On the June 11 episode of 205 Live, he made his WWE debut, defeating Sunil Singh. On the September 28 episode of NXT, Waller unsuccessfully challenged Roderick Strong for the NXT Cruiserweight Championship. On the November 9 episode of NXT, he lost to Solo Sikoa in a triple threat match which also included LA Knight.
On the November 23 episode of NXT, Waller berated the fans, thus turning heel. At NXT WarGames, he teamed up with Bron Breakker, Carmelo Hayes, and Tony D'Angelo as Team 2.0, where they defeated Team Black & Gold (Johnny Gargano, LA Knight, Pete Dunne, and Tommaso Ciampa) in a WarGames match. On the following episode of NXT, Waller brutally attacked Johnny Gargano during his farewell promo where he thanked his fans and friends, thus cementing his heel turn. On the December 21 episode of NXT, he attacked Dexter Lumis after the latter's match, and announced that he would dismantle the old NXT one by one. He then confronted AJ Styles in whom he had criticized on social networks a few days before. At NXT New Years Evil, Styles once again confronted Waller, resulting into a brawl between the pair. The following week on the January 11 episode of NXT 2.0, Waller lost to Styles. After the match, he was attacked by the returning LA Knight. On the February 2 episode of NXT 2.0, Waller, alongside his new "insurance policy" Sanga, interfered in Knight's match against Joe Gacy, causing Knight to lose the match. On March 8, during the NXT Roadblock special, he defeated Knight in a Last Man Standing match, albeit needing help from Sanga to beat the ten-count and get back to his feet.
At NXT Stand and Deliver on April 2, Waller faced Cameron Grimes, Carmelo Hayes, Santos Escobar and Solo Sikoa for the NXT North American Championship, a match which Grimes would win. Waller and Sanga would enter the tag team gauntlet match for the vacant NXT Tag Team Championships, where they were eliminated by the Creed Brothers. Waller then turned on Sanga and defeated him a week later on the April 19 episode of NXT. Waller then briefly feuded with the debuting Nathan Frazer, whom he lost to at NXT Spring Breakin'. Waller would then pursue the NXT North American Championship by tricking Carmelo Hayes into signing a contract. At NXT The Great American Bash, Waller lost to Hayes due to interference from Trick Williams and Wes Lee. Despite this, Waller blamed his loss on Wes Lee and defeated him on the July 26 episode of NXT due to interference from Williams. At NXT Deadline on December 10, Waller won the inaugural Iron Survivor Challenge to become the number one contender for Bron Breakker's NXT Championship. The match was later scheduled for NXT New Year's Evil on January 10, 2023, where the match ended when the middle rope snapped, causing Waller to fall out of the ring and be counted out, allowing Breakker to retain the title. A steel cage match between the two for the title was then subsequently scheduled for NXT Vengeance Day on February 4, which Waller would go on to lose.
As part of the 2023 WWE Draft, Waller was drafted to the SmackDown brand. He made his SmackDown debut on May 19, interviewing AJ Styles on his talk show, The Grayson Waller Effect. At Money in the Bank, Waller interrupted the returning John Cena, where he confronted and attacked Cena before getting hit with an Attitude Adjustment. On the July 7 episode of SmackDown in Madison Square Garden, Waller interviewed Edge on The Grayson Waller Effect. Waller then made his main roster in-ring debut later that night in a losing effort against Edge. On the November 10 episode of SmackDown, Waller and his new tag team partner Austin Theory provoked the newest member of the SmackDown roster Kevin Owens, leading to Owens attacking both Waller and Theory. On the December 15 episode of SmackDown, both Waller and Theory lost in the first rounds of the United States Championship number one contender tournament to NXT's Carmelo Hayes and Owens respectively. Waller returned to NXT at NXT New Year's Evil on January 2, 2024 and was put into a match against Trick Williams for Williams' Iron Survivor Challenge title match opportunity by Hayes. Waller lost the match after interference from Owens. At Elimination Chamber: Perth, held in his home country of Australia, Waller co-hosted the Grayson Waller Effect with Austin Theory, featuring Cody Rhodes and Seth Rollins as his guests. On the February 26 episode of Raw, Waller, in his debut match on the brand, was defeated by Rhodes in the main event.
Waller and Theory went on to win a tag team tournament, defeating The O.C. (Karl Anderson and Luke Gallows) on the March 22 episode of SmackDown and The Street Profits (Angelo Dawkins and Montez Ford) in the finals one week later to qualify for the six-pack ladder match for the Undisputed WWE Tag Team Championship at WrestleMania XL. On night one of WrestleMania XL, Waller retrieved the SmackDown Tag Team Championships for A-Town Down Under to become the new champions, marking Waller's first championship in WWE. On the April 19 episode of SmackDown, the SmackDown Tag Team Championship was renamed as the WWE Tag Team Championship and the team was presented with new title belts by Triple H. On the May 3 episode of SmackDown, Theory and Waller defeated The Street Profits (Angelo Dawkins and Montez Ford) in their first title defense. On the July 5 episode of SmackDown, Waller and Theory lost the titles to #DIY (Johnny Gargano and Tommaso Ciampa), ending their reign at 90 days. A-Town Down Under failed to win the titles in a rematch on the following episode of SmackDown, ending their feud. Waller and Theory returned to NXT on September 24, challenging Fraxiom (Nathan Frazer and Axiom) for the NXT Tag Team Championships, which they accepted. Later that night, they defeated Cedric Alexander and Je'Von Evans following a backstage confrontation. On the October 8 episode of NXT, they failed to win the titles from Fraxiom.
On the December 13 episode of SmackDown, Waller and Theory met with Braun Strowman backstage and asked him to be a guest on The Grayson Waller Effect next week, which Strowman accepted. On the December 20 episode of SmackDown, Waller hosted The Grayson Waller Effect alongside Austin Theory with Braun Strowman as the special guest, which was later interrupted by Carmelo Hayes. On the January 24, 2025 episode of SmackDown, it was announced by SmackDown general manager Nick Aldis that Austin Theory and Grayson Waller were traded to the Raw brand. On the January 28 episode of NXT, A-Town Down Under hosted a special edition of The Grayson Waller Effect with the new NXT Champion Oba Femi as the guest. Femi then offered to defend the NXT Championship in a triple threat match against A-Town Down Under at NXT Vengeance Day on February 15, where Femi retained the title after Waller cost Theory the win. On the July 21 episode of Raw, Waller aligned himself with The New Day after Theory suffered an injury, quietly disbanding A-Town Down Under.