In April, 2018, Theory defeated Marcel Barthel on day four of WrestleMania Axxess.
On August 15, 2019, it was revealed that Theory had signed to WWE and would start reporting to the WWE Performance Center. He would make his NXT debut on the December 25 edition of NXT, accepting an open challenge against Roderick Strong for the NXT North American Championship, where he would lose after tapping to the Stronghold. The following week, Theory would beat Joaquin Wilde, securing his first win on the brand. On the February 19 edition of NXT, Theory would try to attack Tommaso Ciampa during a promo, turning heel in the process. This led to a match between the two on February 26 in which Theory would prove unsuccessful against Ciampa.
Theory appeared on the March 30 episode of Raw, teaming with Angel Garza and Seth Rollins, where they were defeated by Kevin Owens and The Street Profits (Angelo Dawkins and Montez Ford). At WrestleMania 36, Theory would make his WrestleMania debut by teaming with Garza and unsuccessfully challenging Street Profits to their Raw Tag Team Championships. The following night on Raw, Theory and Garza were once again defeated by The Street Profits in a rematch for the titles. After defeating Akira Tozawa on the April 13 episode of Raw, Theory began to be officially managed by Zelina Vega. Vega's other associates, Garza and Andrade, joined him in beating down Tozawa as all three posed together, confirming their status as a faction. On the April 21 Raw, Theory lost to Aleister Black in a Money in the Bank qualifying match.
On the May 18 Raw, Theory was kicked out of Vega's stable after he was attacked by both Andrade and Garza. Later that night, he formally joined Seth Rollins' stable by attacking Aleister Black during Black's match with fellow disciple Murphy. However, following the June 22 episode of Raw, he stopped appearing with Rollins and Murphy.
On the August 26 episode of NXT, Theory returned to television by interrupting Bronson Reed, seemingly moving back to the brand. Theory would begin his time back in NXT as an enhancement talent, often losing his matches to the likes of Reed, Dexter Lumis, and others. Theory would reveal himself as the newest member of the Gargano family at NXT TakeOver: WarGames by helping Johnny Gargano win the NXT North American Championship for a record setting third time. He then appeared on the December 9 episode to reveal he was part of a villainous faction called The Way with Gargano, his wife Candice LeRae, and Indi Hartwell.
On the October 1, 2021 episode of SmackDown, the first night of the 2021 WWE Draft, Theory was drafted to the Raw brand. On the October 4 episode of Raw, Theory made his debut by attacking Jeff Hardy following his loss to Damian Priest. The next week, Theory defeated Jeff Hardy, and followed up with a second win against him on the October 18 episode of Raw. Later, he tried to take a selfie with Rey Mysterio, but was disapproved by Rey’s son, Dominik. Austin challenged Dominik to a match, and he accepted, which Theory won. After Mysterio was injured by Bobby Lashley, Austin replaced Rey for the Raw men’s Survivor Series team at Survivor Series. At the event, Theory would be able to eliminate Sheamus, but was eliminated by Jeff Hardy. The following night on Raw, it was revealed that Theory took "Cleopatra's Egg," a gift given to Mr. McMahon by The Rock, from Mr. McMahon. As a result, McMahon, at first mad at Theory, then told Theory he showed intestinal fortitude and reminded McMahon how he was when younger days. McMahon rewarded Theory a WWE Championship match against Big E in the main event; Theory would lose the match. Austin then started a mini feud with Finn Bálor, where Bálor defeated Theory in a singles match. The next week on Raw, Vince McMahon put Theory in the men's Royal Rumble match. At the Royal Rumble event, Theory came in at number three and lasted twenty-two minutes, scoring one elimination as he and other superstars eliminated Omos, but Theory was ultimately eliminated by AJ Styles. The next night at Raw, Theory faced Kevin Owens in a qualifying match for the WWE Championship match at the Elimination Chamber event, and came out victorious. At Elimination Chamber, Theory failed to win the WWE Championship after being last pinned by Brock Lesnar in a match also involving Seth Rollins, AJ Styles, Riddle and the defending champion Bobby Lashley. Toward the end of the Elimination Chamber match, Lesnar delivered his finishing move, the F-5, throwing Theory from the top of one of the Chamber's pods down to the floor.
On the February 28, 2022 episode of Raw, Theory would attempt to bribe McMahon into having himself accompany him to his interview with Pat McAfee, however this idea was shot down by McMahon. That same week on SmackDown, Theory would appear despite being on the other brand and would confront McAfee, announcing that he would be McAfee's opponent at WrestleMania 38. Theory would end up losing the match, but helped Mr. McMahon defeat McAfee in the subsequent fight that ensued right after. The following night on Raw, Theory teamed up with The Usos to face RK-Bro and United States Champion Finn Balór in a winning effort.
On April 11, in a backstage segment, Theory announced that he and Mr. McMahon had decided that the name Austin didn’t suit him anymore, dropping the first name and going by Theory. On the April 18 episode of Raw, Theory defeated Bálor to become United States Champion for the first time in his career. On the May 9 episode of Raw, Theory defended the title against and lost to Cody Rhodes by disqualification following interference from Seth Rollins, but retained the title. At Hell in a Cell, Theory retained the title against Mustafa Ali. At Money in the Bank, Theory lost the United States Championship to Bobby Lashley via submission, ending his reign at 75 days. Later that night, he was added to and won the 2022 men's Money in the Bank match. At SummerSlam, Theory failed to regain the United States Championship in a rematch against Lashley. Later that night, he made an appearance during the Undisputed WWE Universal Championship Last Man Standing match between Brock Lesnar and Roman Reigns in an attempt to cash in his Money in the Bank contract. However, before he could officially cash in, he was hit with a F-5 by Lesnar. At Clash at the Castle, his ring name was reverted back to Austin Theory. He took part in a six-man tag team match on the pre-show, where his team was defeated. Later that night, he again attempted to cash in his Money in the Bank contract during the main event, only to be knocked out by Tyson Fury before officially doing so. On the November 7 episode of Raw, Theory cashed in his Money in the Bank contract on Seth Rollins for the United States Championship, but lost the match due to Lashley's interference. This made Theory the first wrestler to cash-in for a secondary championship in WWE and the fifth to fail to win the title upon cash-in.
On November 26, at Survivor Series WarGames, Theory defeated Rollins and Lashley in a triple threat match to win the United States Championship for the second time in his career. On the December 5 episode of Raw, Theory retained the title against Mustafa Ali by disqualification after being attacked by the returning Dolph Ziggler. On the January 2, 2023 episode of Raw, Theory retained the title against Rollins, as well as Lashley in a no disqualification match at Raw is XXX on January 23 after interference from the returning Brock Lesnar. At the Royal Rumble on January 28, Theory entered the Royal Rumble match at number twenty-five, but was eliminated by eventual winner Cody Rhodes. At Elimination Chamber on February 18, Theory successfully defended the title against Bronson Reed, Damian Priest, Johnny Gargano, Montez Ford, and Seth Rollins in an Elimination Chamber match, marking the first time the title was contested inside the namesake structure. In a press conference after the event, Theory issued an open challenge for the title for the following episode of Raw, which was answered by Edge. On the following episode of Raw, Theory retained the title against Edge after interference from Finn Bálor. On the March 6 episode of Raw, Theory challenged the returning John Cena to a match for the title at WrestleMania 39, which Cena accepted. On Night One of the event, Theory defeated Cena to retain the title.
As part of the 2023 WWE Draft, Theory was drafted to the SmackDown brand. At Backlash, Theory retained the title against Bronson Reed and Lashley. On the May 12 episode of SmackDown, he faced Bobby Lashley and Sheamus in a triple threat match for the World Heavyweight Championship tournament, but was pinned by Lashley in the first round. Two weeks later on the May 26 episode of SmackDown, Theory retained the United States Championship against Sheamus after interference from Pretty Deadly (Elton Prince and Kit Wilson). He then retained the title against Jey Uso and Sheamus on the June 9 and July 7 episodes of SmackDown, respectively. On the August 11 episode of SmackDown, Theory was scheduled to defend the title against Santos Escobar, but was rendered unable to compete after Theory attacked him backstage. Later that night, Theory was defeated by Escobar’s Latino World Order stablemate Rey Mysterio in an impromptu title match, ending his second reign at 258 days. The following week, he defeated LA Knight after interference from The Miz to become the number one contender to the United States Championship, but failed to regain the title back from Mysterio at Payback.
On the September 15 episode of SmackDown, Theory was involved in a segment with Pat McAfee before The Rock returned to attack Theory with the People's Elbow. During this time, he began a partnership with Grayson Waller known as A-Town Down Under. They had their first win as a team against the Brawling Brutes (Butch and Ridge Holland) on the September 22 episode of SmackDown. On the December 15 episode of SmackDown, Theory and Waller both lost in the first rounds of the United States Championship number one contender tournament to Kevin Owens and NXT's Carmelo Hayes. On the January 12, 2024 episode of SmackDown, Theory's match against Hayes ended in a no-contest after both of them landed on their heads from the top rope. Theory went on to defeat Hayes in the following week's rematch. In the 2024 Royal Rumble, Theory entered the match at number thirteen before being eliminated by eventual winner Cody Rhodes.
On February 24, at Elimination Chamber: Perth, Theory co-hosted The Grayson Waller Effect with Waller featuring Cody Rhodes and Seth Rollins. After imitating The Rock, Cody and Seth attacked Theory while Waller did not get involved, teasing the dissolution of A-Town Down Under. Nevertheless, Theory and Waller entered 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 a week later to qualify for the six-pack ladder match for the Undisputed WWE Tag Team Championships 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. 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, Theory and Waller lost the titles to #DIY (Johnny Gargano and Tommaso Ciampa), ending their reign at 90 days. They failed to regain the titles in a rematch on the following episode of SmackDown, ending their feud.
After a promo on social media the previous week, Theory and Waller returned to NXT on September 24, challenging Fraxiom (Nathan Frazer and Axiom) for the NXT Tag Team Championship, 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.