Sullivan reported to the WWE Performance Center by October 2014, and made his professional wrestling debut in a showcase match at WrestleMania Axxess prior to WrestleMania 31 in March 2015, defeating Marcus Louis. He then made sporadic appearances at NXT live events over the following two years.
Lars made his television debut, under the name Dylan Miley, on the April 12, 2017 episode of NXT, teaming with Michael Blais in a loss to DIY. Following the match, Miley attacked Blais. In May 2017, he adopted the ring name Lars Sullivan. After several similar tag team appearances which resulted in him attacking his partner, Sullivan made his first appearance as a singles performer on the August 23 episode of NXT, attacking No Way Jose before a scheduled match. His first televised singles match and victory took place on the September 6 episode of NXT, defeating three men in a three-on-one handicap match.
Following weeks of squash matches, Kassius Ohno asked General Manager William Regal for a match against Sullivan at NXT TakeOver: WarGames on November 18, which Sullivan won. In December, Sullivan was involved in a tournament to determine NXT Champion Andrade "Cien" Almas' opponent for NXT TakeOver: Philadelphia. On the December 20 episode of NXT, Sullivan defeated Roderick Strong to advance to the tournament finals. The following week, he was unsuccessful in a number one contenders fatal four-way match also involving Aleister Black, Johnny Gargano and Killian Dain. At the NXT TakeOver: New Orleans on April 7, 2018, Sullivan took part in a six-man ladder match for the NXT North American Championship, which was won by Adam Cole. On the May 16 episode of NXT, Sullivan attacked both Ricochet and Velveteen Dream during their match. The following week, he defeated both men in a handicap match. On the May 30 episode of NXT, Sullivan revealed that he would challenge NXT Champion Aleister Black for his championship at NXT TakeOver: Chicago II. He was unsuccessful in his challenge. It also marked his first loss via pinfall in NXT.
On November 18, during the Survivor Series pre-show, vignettes aired for Sullivan's main roster debut. Throughout the next several weeks, vignettes aired on both Raw and SmackDown to hype Sullivan's debut, with the commentators remarking that both brands were attempting to sign Sullivan. Sullivan was scheduled to appear on the January 14, 2019 episode of Raw, alongside other NXT wrestlers but reportedly walked out during the program, citing an anxiety attack.
Sullivan eventually debuted on the main roster on the April 8 episode of Raw by attacking Kurt Angle, who had retired the previous night. WWE pushed him as a monster heel, attacking several high profile wrestlers like The Hardy Boyz and Rey Mysterio among others. On April 16, he became part of the SmackDown brand during the Superstar Shake-up. He then started a storyline with the three members of Lucha House Party. At Super Showdown, Sullivan competed in a three-on-one handicap match, which he won by disqualification. On the June 10 episode of Raw, Sullivan defeated the Lucha House Party in a 3-on-1 handicap elimination match. However, he injured his knee during the match and was estimated to be out of action between six and nine months.
Sullivan made his return on the October 9, 2020 episode of SmackDown, attacking The Miz, Jeff Hardy and Matt Riddle.
On February 2, 2021, it was announced that Sullivan had been released from his contract.