Prior to signing with WWE, Kross appeared on the February 16, 2015 episode of Raw, where he teamed with Darren Young to face The Ascension (Konnor and Viktor) in a match that resulted in a no contest.
On the February 4, 2020 episode of WWE Backstage, it was confirmed that Kross had signed with WWE. In the following weeks, mysterious video packages were shown that were later to be revealed as Kross. On the April 8 edition of NXT, Kross, along with Scarlett, could be seen in a car watching Johnny Gargano, following his match with Tommaso Ciampa. The following week, Kross debuted, attacking Ciampa. Later that night, it was revealed that Kross would be known as Karrion Kross. On the May 6 episode of NXT, Kross made his debut defeating Leon Ruff in a squash match. At NXT TakeOver: In Your House, Kross defeated Tommaso Ciampa via technical submission. At NXT TakeOver: XXX, he defeated Keith Lee to win the NXT Championship for the first time. However, the following day on August 23, 2020, WWE announced that Kross had suffered a separated shoulder during the match, and on the August 26 episode of NXT, he vacated the title; this ended his reign at only four days, making it the shortest in the title's history. On the December 9 episode of NXT, Kross would return by attacking Damian Priest. At NXT New Year's Evil, he defeated Priest. On the February 3 episode of NXT, Kross attacked Legado del Fantasma members Raul Mendoza and Joaquin Wilde, before threatening the leader, NXT Cruiserweight Champion Santos Escobar. Later in the night, Kross would confront 2021 Royal Rumble match winner Edge. On the February 24 edition of NXT, Karrion Kross defeated Santos Escobar in a street fight. Kross confronted the NXT Champion Finn Bálor after he successfully defended his title against Adam Cole on the March 10 edition of NXT. The following week, Bálor and Kross would unsuccessfully challenge Oney Lorcan and Danny Burch for the NXT Tag Team Championship after Lorcan pinned Bálor, after he inadvertently knocked down Scarlett, leading to Kross turning on Bálor, and also marking Kross' first loss in NXT. He would then go on to defeat Balor at NXT TakeOver: Stand and Deliver to win the NXT Championship for the second time. On the May 25 episode of NXT, Kross defeated Balor again to retain the title. On June 13 at NXT TakeOver: In Your House, Kross successfully retained the championship by defeating Pete Dunne, Kyle O'Reilly, Johnny Gargano, and Adam Cole in a fatal five-way match. On the July 13 episode of NXT, Gargano faced Kross for the NXT Championship with Samoa Joe as guest referee, where he successfully retained.
On the July 19 episode of Raw, Kross made his main roster debut where he was defeated by Jeff Hardy by rollup, marking his first ever pinfall loss in WWE. On the following week's Raw, Kross defeated Keith Lee, but lost to him the next week. At NXT TakeOver: 36, Kross lost for the first time in NXT, losing his NXT Championship in the main event to Samoa Joe after a heated rivalry. After he lost the NXT Championship, Kross was transferred full-time to Raw and, on the August 23 episode, Kross debuted a new look where he wore a gladiator helmet during his entrance. On November 4, Kross, along with Scarlett, were both released from their WWE contracts.
On the August 5, 2022 episode of SmackDown, Kross made his unannounced return to WWE alongside Scarlett, attacking Drew McIntyre at the end of the show and officially joining the SmackDown brand in the process. On the September 23 episode of SmackDown, McIntyre challenged Kross to a strap match at Extreme Rules, which he accepted. At the event on October 8, Kross defeated McIntyre with the Kross Hammer after interference by Scarlett. On the SmackDown following Extreme Rules, Kross was scheduled to compete in a fatal four-way to determine the number one contender for the Intercontinental Championship; however, as the show went on the air, Kross was seemingly involved in a car accident and as he was receiving help, he was attacked by McIntyre, rendering him unable to compete. Kross was replaced by Rey Mysterio, who went on to win the ensuing four-way. At Crown Jewel on November 5, Kross lost to McIntyre in a steel cage match when McIntyre escaped the cage before he did, ending their feud. Beginning in December 2022, Kross began a feud with Rey Mysterio. On the January 27 episode of SmackDown, Kross lost to Mysterio after a roll up pin. The next night at Royal Rumble, Kross entered his first Royal Rumble match at number seven, but was eliminated by McIntyre. On the February 24 episode of SmackDown, Kross was announced as part of a fatal five-way match with former rival Drew McIntyre, Sheamus, LA Knight, and Kofi Kingston to qualify for a match with Gunther for the Intercontinental Championship at WrestleMania 39. The following week on SmackDown, Kross failed to win the match.
As part of the 2023 WWE Draft, Kross and Scarlett were drafted to the SmackDown brand. Throughout the summer, Kross feuded with AJ Styles, with the two trading victories against each other on various episodes of SmackDown. The feud concluded on the August 11 episode, where Styles emerged victorious. After a four-month hiatus, Kross made his return on the December 8 Tribute to the Troops special episode of SmackDown, taking part in the United States Championship number one contender tournament, where he would lose in the first round to Bobby Lashley. On the December 22 episode of SmackDown, Kross and Scarlett appeared in a vignette, stating that they had formed a new alliance. On SmackDown New Year's Revolution, Kross, accompanied by Scarlett, attacked Bobby Lashley and the Street Profits alongside the Authors of Pain and Paul Ellering, who officially made their return to WWE, confirming their alliance in the process. The following week, the stable was officially named The Final Testament. Kross entered the 2024 Royal Rumble match at number eight, where he and Bobby Lashley eliminated each other to continue their feud. On night two of WrestleMania XL on April 7, Kross, along with Akam and Rezar, lost to Lashley and The Street Profits in a six-man Philadelphia street fight. On the following episode of NXT, Kross made his return to the brand alongside The Final Testament.
During night two of the 2024 WWE Draft on April 29, Kross was drafted to Raw along with the rest of The Final Testament. He, along with the rest of the Final Testament, soon began feuding with The New Day, attempting to drive a wedge between Xavier Woods and Kofi Kingston by claiming that Kingston had been preventing from Woods from achieving success as a singles competitor, leading to several bouts between the factions. The feud concluded on the August 19 episode of Raw, when The Final Testament lost to The New Day and Odyssey Jones in a six-man tag team match.