On May 15, 2014, Bálor (as Prince Devitt) was reported to have signed with WWE and would join NXT, the promotion's developmental territory. On September 24, his new ring name was revealed to be Finn Bálor. Bálor made his NXT debut the next day, aiding Hideo Itami against The Ascension (Konnor and Viktor). In his debut match on October 23, he and Itami defeated Justin Gabriel and Tyson Kidd. After feuding with The Ascension, Bálor and Itami defeated them at NXT TakeOver: R Evolution on December 11, where he also debuted his signature body paint. Bálor then entered an NXT Championship number one contender's tournament, defeating Curtis Axel in the first round, Itami in the semi-finals, and Adrian Neville in the finals at NXT TakeOver: Rival. Bálor received his title match against Kevin Owens on March 25, but was unsuccessful.
After defeating Tyler Breeze on May 20 at NXT TakeOver: Unstoppable, Bálor received another title shot against Owens and at The Beast in the East in Tokyo, Japan and defeated Owens to win the NXT Championship. Bálor defeated Owens in a ladder match to retain the title at NXT TakeOver: Brooklyn on August 22. Bálor later entered the Dusty Rhodes Tag Team Classic tournament in August, pairing with Samoa Joe and defeating The Lucha Dragons (Sin Cara and Kalisto) in the first round, Enzo Amore and Colin Cassady in the quarter-finals, Dash Wilder and Scott Dawson in the semi-finals and Baron Corbin and Rhyno in the finals at NXT TakeOver: Respect to win the tournament. On the November 4 episode of NXT, Bálor defended the NXT Championship against Apollo Crews, but the match ended in a no contest after Baron Corbin interfered, attacking both men. Joe came out and chased Corbin away before turning on Bálor and attacking him, which ignited a feud between the two. At NXT TakeOver: London, Bálor defeated Joe to retain the title. On April 1 at NXT TakeOver: Dallas, Bálor retained the NXT Championship in a rematch with Joe. On April 17, Bálor became the longest reigning NXT Champion in history by surpassing Neville's previous record of 287 days. On April 21, Bálor lost the NXT Championship to Joe at a live event in Lowell, Massachusetts, ending his reign at 292 days. At NXT TakeOver: The End, Bálor lost to Joe in the first ever NXT steel cage match in a rematch for the NXT Championship, marking his first TakeOver loss, and subsequently his first loss under his "Demon" persona. Bálor wrestled his final NXT match on July 30, teaming with Shinsuke Nakamura to defeat Bobby Roode and Samoa Joe.
On July 19, 2016, Bálor was drafted to the Raw brand as the fifth overall pick in the 2016 WWE draft, later declaring that he should have been the first pick. He made his first appearance for the brand on the July 25 episode of Raw, where he won the right to compete for the newly established WWE Universal Championship at SummerSlam by first defeating Rusev, Cesaro, and Kevin Owens in a fatal four–way match and then defeating Roman Reigns, who had won a similar fatal four–way match. At SummerSlam, Bálor defeated Seth Rollins to become the inaugural Universal Champion and win his first world title. The day after, WWE reported that he sustained a shoulder injury during the match. Because of this, it was expected that Bálor would be out four to six months, thus Raw General Manager Mick Foley announced that Bálor would be relinquishing his newly won WWE Universal Championship due to his injury. While recovering from his injury, Bálor would appear on the WWE Network–exclusive United Kingdom Championship Tournament event.
On February 22, 2017, Bálor returned to NXT to assist Shinsuke Nakamura, who was outnumbered by Andrade "Cien" Almas and Bobby Roode. On March 10, Bálor made his in–ring return at a live event in a six–man tag team match, teaming with Chris Jericho and Sami Zayn to defeat Kevin Owens, Samoa Joe and Triple H. Bálor made his television return after WrestleMania 33 on the April 3 episode of Raw, where he teamed with his former rival Seth Rollins as the two defeated Kevin Owens and Samoa Joe in a tag team match. Over the following weeks, Bálor would score victories over Jinder Mahal and Curt Hawkins while also receiving an ominous warning from Bray Wyatt and confronting Luke Gallows and Karl Anderson. On June 4 at Extreme Rules, Bálor competed in a fatal five–way extreme rules match to determine the number one contender for Brock Lesnar's Universal Championship, in which Bálor was defeated after he passed out from Joe's coquina clutch.
In the weeks leading up to SummerSlam, Bálor began a feud with Elias Samson after he inadvertently interrupted one of his performances. This led to a match between the two, where Samson defeated Bálor in a no disqualification match after interference provided by Bálor's rival Bray Wyatt. In a match between Bálor and Wyatt, Wyatt was able to score a victory over Bálor. The two faced off in a rematch, at SummerSlam, where Bálor returned his Demon King character and defeated Wyatt. The feud between the two continued throughout the following months, that led to another match, at No Mercy where Bálor once again defeated Wyatt. The two were also set to face in a rematch between Bálor's Demon King and Wyatt's Sister Abigail at TLC, however, Wyatt was ruled out for the event on 20 October due to an illness, and was instead replaced by AJ Styles. Although he defeated Styles, the two showed mutual respect and gestured the "Too Sweet" hand symbol to each other following the match. In November, at the Survivor Series event, Bálor took part of the traditional interbrand five–on–five elimination match as part of Team Raw where he was the fifth man eliminated by Randy Orton, although his team was ultimately victorious.
On the January 1, 2018 episode of Raw, Bálor teamed with Luke Gallows and Karl Anderson as the trio defeated Elias and The Miztourage (Curtis Axel and Bo Dallas) in a six–man tag team match. The following week, the trio officially reunited and formed the Bálor Club. That same night, they defeated Intercontinental Champion Roman Reigns and Raw Tag Team Champions Seth Rollins and Jason Jordan in a six–man tag team match. On January 28, at the 2018 Royal Rumble, Bálor entered his first royal rumble match at number two, lasting over fifty-seven minutes and eliminating four participants before being eliminated by John Cena. A month later, Bálor competed for the first time in an elimination chamber match at the namesake pay–per–view, where he entered last but was the second participant eliminated by Braun Strowman. Throughout March and April, Bálor started a short feud with The Miz and Seth Rollins over the Intercontinental Championship, which led to a triple threat match at WrestleMania 34 (which was also Bálor's WrestleMania debut) where Rollins was the one who captured the belt. Three weeks later, at the Greatest Royal Rumble event, Bálor once again unsuccessfully challenged for the Intercontinental Championship in a fatal four–way ladder match against Rollins, Miz and Samoa Joe. In June, Bálor competed in a Money in the Bank ladder match for the first time in his career, however, the match was ultimately won by Braun Strowman.
After the Money in the Bank pay–per–view, Bálor engaged in a feud with Baron Corbin, whom he defeated on two different occasions—in July (at the Extreme Rules event) and in August (at the SummerSlam event under his "Demon King" persona). One night after SummerSlam, on the August 20 episode of Raw, Bálor was finally awarded with his Universal Championship rematch against the newly crowned champion Roman Reigns, however he was unsuccessful in regaining the title. Throughout the next few months, Bálor would compete in various matches against competitors such as Jinder Mahal and Bobby Lashley. In November, Bálor started a feud with Drew McIntyre who acted as if he was trying to save him from an attack by Lashley only to attack him himself. During that time, Bálor was announced as part of Team Raw for the traditional interbrand five–on–five elimination match at Survivor Series, where he was the first man from the team to be eliminated by Rey Mysterio.
On December 16, at the TLC: Tables, Ladders & Chairs event, Bálor gained revenge over McIntyre as he defeated him in a singles match after interference by his former ally Dolph Ziggler. After the match, Ziggler would attack Bálor backstage after the latter implied that he didn't need his help to beat McIntyre. This sparked a match between the two, that took place the following night on Raw, which ended in a no contest as McIntyre would attack both of them. On the December 24 edition of Raw, Bálor defeated Ziggler and McIntyre a triple threat match, ending the feud. On January 12, 2019 at NXT UK TakeOver: Blackpool, Bálor made a surprise appearance as Travis Banks's replacement in his match against Jordan Devlin, as Banks was attacked by Devlin earlier that day. Bálor subsequently defeated Devlin with the Coup de Grâce. On the January 14 episode of Raw, after Braun Strowman was removed from his Universal Championship title match against Brock Lesnar at the Royal Rumble after damaging Vince McMahon's limo, Bálor was booked in a fatal four-way match against Drew McIntyre, John Cena and Baron Corbin to determine who would take Strowman's place against Lesnar at the pay-per-view, in which Bálor emerged victorious after pinning Cena. At the Royal Rumble pay-per-view, Bálor was unsuccessful in defeating Lesnar after tapping out to the Kimura Lock. After the match, Lesnar attacked Bálor with three F-5s.
On the January 28 edition of Raw, Bálor was interrupted by Intercontinental Champion Bobby Lashley and Lio Rush while Bálor was addressing his loss, and was subsequently attacked by Lashley, sparking a feud between the two. On February 17 at the Elimination Chamber pay-per-view, Bálor defeated Lashley and Rush in a two-on-one handicap match to capture the Intercontinental Championship, after Bálor pinned Rush. Bálor faced Lashley on the 11 March edition of Raw in an Intercontinental championship rematch. Bálor lost the match and the championship to Lashley following interference from Lio Rush. On the March 25 edition of Raw, Bálor defeated Lashley and Jinder Mahal in a handicap match to gain a rematch for the Intercontinental title at WrestleMania 35. Bálor, using his demon persona, went on to defeat Lashley at WrestleMania and regained the Intercontinental championship. The following night on Raw, Bálor defeated the returning Sami Zayn in an impromptu title defense. In the 2019 WWE Superstar Shake-up, Bálor moved to SmackDown, making his debut on the April 16 edition of SmackDown Live, defeating Ali in a non-title match. The following week on April 23, Bálor was victorious in a rematch against the returning Andrade. At Super Showdown, Balor defended his title against Andrade with his demon persona. On the Extreme Rules kick-off show, Bálor lost the championship to Shinsuke Nakamura. The next night on Raw, he was attacked by a returning Bray Wyatt now going under the gimmick of The Fiend. At SummerSlam, Balor lost to The Fiend.
After a near two-month absence from television, Bálor made his return to WWE programming on the October 2 episode of NXT, confronting NXT Champion Adam Cole following the latter's title defense against Matt Riddle and officially announced his return to NXT. On the October 23 episode of NXT, Bálor turned heel for the first time in his WWE career, when he aided The Undisputed Era in attacking Johnny Gargano and Tommaso Ciampa. On the November 13 episode of NXT, Bálor insulted the NXT roster by referring to them as “all boys who can't take a beating”, specifically mentioning Gargano and Matt Riddle. Riddle then attacked Bálor, who retreated. Later that night, Riddle assisted his then-WarGames partners Keith Lee and Ciampa against their scheduled opponents, The Undisputed Era. However, Bálor appeared and attacked Riddle. As a result, a match between the two was scheduled for NXT TakeOver: WarGames, where Bálor emerged victorious. In February, Bálor defeated a returning Gargano at TakeOver: Portland.
On the April 22, 2020 episode of NXT, Bálor was scheduled to have a match against Velveteen Dream, but Bálor was knocked out by someone unknown before he could leave his dressing room. Two weeks later, Bálor concluded that there was a "snake hiding in the long grass back there" and that whoever attacked him wanted a push. Bálor assured the audience that it won't be a push, but rather a squash. Later that night, he got into an altercation with Cameron Grimes, which resulted in a match between the two the next week. During the match, Damian Priest cost Bálor the match by using a nightstick behind the referee's back. Priest then revealed that he was the one who knocked out Bálor three weeks prior, thus turning Bálor face once again. At NXT TakeOver: In Your House, Bálor defeated Priest. Two months later at NXT TakeOver: XXX, Bálor defeated Timothy Thatcher. On September 8 on NXT Super Tuesday II, Bálor defeated Adam Cole to win vacant NXT Championship for the second time. At NXT TakeOver: 31, Bálor made a successful title defense against Kyle O'Reilly. It was revealed he had suffered a broken jaw during the match and was forced to undergo surgery. Bálor made his return on the December 9 episode of NXT, where he was confronted by O'Reilly, Pete Dunne, Damian Priest and Scarlett (on behalf of Karrion Kross). Balor would then successfully defend the championship in a rematch against O'Reilly at NXT: New Year's Evil. He would also defend the championship against Pete Dunne and Adam Cole at NXT TakeOver: Vengeance Day and on the March 10 episode of NXT respectively, both times successfully defending it. At NXT TakeOver: Stand and Deliver, Bálor lost the championship to Karrion Kross, ending his second reign at 212 days. Balor would face Kross in a rematch for the title on the May 25 episode of NXT, but was defeated.
After a two-month hiatus from television, Bálor made his return to the main roster on the July 16 episode of SmackDown, confronting and attacking Sami Zayn. The following episode of SmackDown, Bálor defeated Zayn in a match and later the same night, would challenge WWE Universal Champion Roman Reigns to a championship match, with the latter accepting. The following week, a contract signing was held for the match, but before Bálor could sign the contract, he was attacked by Baron Corbin, resulting in John Cena signing it instead. This led to match between Balor and Corbin the following week on SmackDown, where Bálor emerged victorious. Bálor would eventually face Reigns for the championship on the September 3 episode, but failed to win the title following interference from The Usos. Following the match, a red light appeared over Bálor, teasing the return of his Demon King persona. The following episode of SmackDown, Bálor appeared as his Demon King persona for the first time since June 2019 and subsequently it was announced that he would face Reigns in a rematch for the championship at Extreme Rules. At the event, The Demon lost to Reigns.
As part of the 2021 Draft, Bálor was drafted to the Raw brand. In October, Bálor entered the King of the Ring tournament, where he defeated Cesaro in the first round, and Sami Zayn in the semifinals, but lost to Xavier Woods in the finals at Crown Jewel. On the February 28, 2022 edition of Raw, Bálor defeated Damian Priest to win the United States Championship for the first time in his career. On the April 18 edition of Raw, Bálor would lose the United States Championship to Theory, ending his reign at 49 days.
At Hell in a Cell on June 5, Bálor teamed with AJ Styles and Liv Morgan to face The Judgment Day (Edge, Damian Priest and Rhea Ripley) in a six-person mixed tag team match, but lost. On the June 6 episode of Raw, Edge introduced Bálor as the newest member of The Judgment Day. However, Bálor, Priest and Ripley suddenly attacked Edge, kicking him out of the group and turning Bálor heel for the first time since May 2020. Bálor and Priest lost to The Mysterios (Rey and Dominik Mysterio) on July 30 at SummerSlam and at Clash at the Castle on September 3, they lost to Rey and Edge, where Dominik turned heel on both of them to join The Judgment Day. At Extreme Rules on October 8, Bálor defeated Edge in an "I Quit" match. On November 5, at Crown Jewel, Bálor, Priest, and Dominik defeated The O.C. (AJ Styles, Luke Gallows, and Karl Anderson) in a six-man tag team match. Three weeks later at Survivor Series WarGames on November 26, Bálor lost to AJ Styles, ending their feud. At the Royal Rumble on January 28, 2023, Bálor entered the Royal Rumble match at number twenty, but was eliminated by the returning Edge. Afterwards, Bàlor and Priest eliminated Edge with help from Dominik. At Elimination Chamber on February 18, Bálor and Ripley lost to Edge and Beth Phoenix in a mixed tag team match. On Night Two of WrestleMania 39 on April 2, Bálor, in his "Demon" persona, lost to Edge in a Hell in a Cell match.
Bálor would then be selected amongst a group of Raw superstars to participate in a tournament to crown a new World Heavyweight Champion. On the May 8 episode, Bálor would defeat Cody Rhodes and The Miz in a triple threat match, before facing Seth "Freakin" Rollins in a singles match in a losing effort. This led to a feud between him and Rollins, who had become the inaugural World Heavyweight Champion after defeating AJ Styles at Night of Champions. Bálor would cite how Rollins cost him the Universal Championship from his injury back in 2016 as a major reason for going after him on the June 12 episode of Raw before challenging him to a title match at Money in the Bank, to which Rollins accepted. At the pay-per-view, Bálor would be unsuccessful in defeating Rollins for the title after an unintended distraction by Damian Priest, who had just won the Money in the Bank ladder match earlier during the night and seemingly had intentions of cashing in, would cause him to lose the match. At SummerSlam on August 5, Bálor failed to win the title from Rollins despite interference from Priest, Dominik and Ripley.
At Payback on September 2, Bálor and Priest defeated Kevin Owens and Sami Zayn to win the Undisputed WWE Tag Team Championships after interference from Dominik, Ripley and JD McDonagh. With this win, Bálor became the twenty-fourth WWE Grand Slam Champion. On the September 25 episode of Raw, Bálor and Priest defeated Owens and Zayn in a rematch to retain their titles. At Fastlane, Bálor and Priest lost the Undisputed WWE Tag Team Championships to Cody Rhodes and Jey Uso after JD McDonagh accidentally cost Priest and Bálor the match. They regained the titles from Rhodes and Uso on the October 16 episode of Raw. On the November 13 episode of Raw, Priest and Bálor defeated Rhodes and Uso in a rematch to retain their titles after interference from Drew McIntyre. On the November 24 episode of SmackDown, Priest and Bálor successfully defended their titles against The Street Profits (Angelo Dawkins and Montez Ford). The next night, at Survivor Series WarGames, Bálor participated in his first WarGames match alongside The Judgment Day (which now included McDonagh) and McIntyre against Rhodes, Rollins, Uso, Zayn and a returning Randy Orton in a losing effort. On the December 18 episode of Raw, Bálor and Priest successfully defended their titles against the Creed Brothers (Brutus Creed and Julius Creed). At the Royal Rumble on January 27, Bálor entered the Royal Rumble match at number fourteen, eliminating Carmelo Hayes alongside Dominik before being eliminated by Bron Breakker. On the following episode of Raw, Priest and Bálor defeated DIY (Johnny Gargano and Tommaso Ciampa) to retain their titles. At Elimination Chamber: Perth on February 24, Bálor and Priest successfully defended the titles against New Catch Republic (Pete Dunne and Tyler Bate). On night one of WrestleMania XL, Bálor and Priest lost the SmackDown Tag Team Championships to A-Town Down Under (Austin Theory and Grayson Waller) and the Raw Tag Team Championships to The Awesome Truth (The Miz and R-Truth) respectively in a six-pack tag team ladder match, ending their second reign with the titles at 173 days.
After WrestleMania XL, Bálor would begin teaming with McDonagh as Priest won the World Heavyweight Championship. They helped Priest retain his title against Jey Uso at Backlash France despite Priest asking them not to interfere. On the May 13 episode of Raw, Bálor and McDonagh won a fatal four-way match to become the number one contenders to The Awesome Truth's newly renamed World Tag Team Championships after interference from Carlito, who began inserting himself into the group. One week later, Bálor and McDonagh failed to win the titles after Braun Strowman prevented Carlito from interfering. On the June 24 episode of Raw, Bálor and McDonagh defeated Miz and Truth to win the titles after interference from Rhea Ripley's former rival, Liv Morgan. At SummerSlam on August 3, Bálor turned on Priest, costing him his World Heavyweight Championship against Gunther. After several brawls and altercations between the new Judgment Day (now consisting of Balor, Mysterio, Morgan, McDonagh and Carlito) and the Terror Twins (Priest and Ripley), a match between Bálor and Priest was set for Bad Blood on October 5, which Bálor lost despite interference from Carlito and McDonagh.