Cass, as Colin Cassady, debuted on NXT television on June 5, 2013 with a loss to Mason Ryan. Cassady then formed a tag team with Enzo Amore, who had also previously lost to Ryan, and they labelled themselves "the realest guys in the room". Ryan defeated Amore and Cassady in consecutive singles matches, but lost to them in a handicap match. Ryan ultimately had the last laugh when he caused Amore and Cassady to be attacked by Tons of Funk (Brodus Clay and Tensai). Amore and Cassady then feuded with Sylvester Lefort and his stable of Alexander Rusev and Scott Dawson. On the September 25 episode of NXT, Amore and Cassady participated in a gauntlet match for a future title shot at the NXT Tag Team Championship; they started the match and defeated Tyler Breeze and CJ Parker, then beat Rusev and Dawson, before falling to their final opponents, The Ascension (Conor O'Brian and Rick Victor). Amore suffered a legit broken leg in November 2013, which led Cassady to try his hand at singles competition, feuding with Aiden English. Amore returned on the June 26, 2014 episode of NXT, saving Cassady from an attack from Sylvester Lefort and Marcus Louis.
In early August, Amore and Cassady participated in a NXT Tag Team Championship number one contenders tournament, and they defeated Jason Jordan and Tye Dillinger in the first round, but were eliminated by The Vaudevillains (Aiden English and Simon Gotch) in the second round. Lefort and Louis then renewed their rivalry with Amore and Cassady by attacking them and shaving Amore's beard and, as a result, Amore challenged Lefort to a match with the loser's hair as forfeit at NXT TakeOver: Fatal 4-Way on September 11, where Amore won the match, but Lefort ran away, leaving his partner Louis to lose his hair at the hands of Amore and Cassady.
Amore and Cassady proceeded to form an alliance with the debuting Carmella after the duo had accidentally cost Carmella her hairdressing job as per the storyline, causing her to demand to get a job as a wrestler. Carmella had her televised in-ring debut on the October 16 episode of NXT. In March 2015, Amore and Cassady began a rivalry with then-NXT Tag Team Champions Blake and Murphy, with the champions insulting Amore and Cassady while attempting to woo Carmella. Amore and Cassady defeated The Lucha Dragons (Kalisto and Sin Cara) in a number one contender's match for a title opportunity against Blake and Murphy. Amore and Cassady received their title match at NXT TakeOver: Unstoppable, but they lost after Alexa Bliss interfered. At NXT TakeOver: London, Amore and Cassady challenged Dash and Dawson for the NXT Tag Team Championship in a losing effort. They were granted another title opportunity against Dash and Dawson in March 2016 at Roadblock, which Amore and Cassady lost.
On the Raw after WrestleMania 32, Amore and Cassady made their main roster debuts and confronted The Dudley Boyz. On the April 14 episode of SmackDown, Amore and Cassady defeated The Ascension in their main roster debut match, advancing in the WWE Tag Team Championship number one contenders tournament. In the finals, Amore and Cassady faced The Vaudevillains at Payback, with the match ending in a no contest after Amore suffered a legitimate concussion during the match. In May 2016, Cassady's ring name was changed to Big Cass. On the June 6 episode of Raw, during the match between Enzo and Cass and The Vaudevillains, Cass witnessed English attempting to injure Enzo in a similar manner of what Gotch did to him at Payback. This led to Cass attacking The Vaudevillains, including executing an East River Crossing on Gotch. At Money in the Bank, Enzo and Cass competed in a fatal four-way tag team match also involving The New Day, Luke Gallows and Karl Anderson and The Vaudevillains for the WWE Tag Team Championship, which they lost. On the July 4 episode of Raw, Enzo and Cass helped John Cena from an attack by The Club, involving themselves in the feud between Cena and The Club. At Battleground, Enzo and Cass teamed with Cena to defeat The Club in a six-man tag team match.
At the 2016 WWE draft on July 19, Enzo and Cass were drafted to Raw. Following Battleground, Enzo and Cass feuded with Chris Jericho and Kevin Owens, losing to them at SummerSlam. On the August 22 episode of Raw, Cass defeated United States Champion Rusev by countout to qualify for the four-way elimination match for the WWE Universal Championship. In that match, the biggest of his career, which also included Kevin Owens, Roman Reigns and Seth Rollins, Cass was eliminated by Owens, marking the first time he was pinned on the main roster.
On the November 7 episode of Raw, Enzo and Cass joined Team Raw for the ten-on-ten Survivor Series tag team elimination match at Survivor Series, which Team Raw won. Afterwards, Enzo and Cass started a rivalry with Rusev. On the December 5 episode of Raw, Enzo was lured into a trap by Lana and Rusev. Big Cass lost to Rusev via countout on the Roadblock: End of the Line kickoff show. Cass lost a handicap match against Rusev and Jinder Mahal before teaming up with Enzo to defeat the duo twice, ending the feud.
Cass entered the 2017 Royal Rumble match at number one and was eliminated by Braun Strowman. At Fastlane, Enzo and Cass unsuccessfully challenged Luke Gallows and Karl Anderson for the Raw Tag Team Championship. At WrestleMania 33, Enzo and Cass faced Gallows and Anderson, Cesaro and Sheamus and the returning Hardy Boyz in a fatal four-way ladder match for the Raw Tag Team Championship, with the Hardy Boyz emerging victorious.
On the Raw following Extreme Rules, Cass was mysteriously attacked in the same way Enzo Amore had been before. In the following weeks, Cass accused Big Show of being his attacker, which Big Show denied. On the June 19 episode of Raw, it was revealed that Cass was behind the attacks on Amore and that he faked his own attack in order to lure away suspicion. Cass admitted to this, expressing his frustration during his time teaming with Amore, and attacked him with a big boot, disbanding the team and turning heel for the first time in his WWE career. On July 9 at Great Balls of Fire, he defeated Amore. The following night on Raw, Cass was attacked by Big Show; this action sparked a feud between Cass and Big Show. On July 31 episode of Raw, Cass defeated Big Show by disqualification when Amore attacked Cass during the match. At SummerSlam, Cass defeated Big Show with Amore being suspended above the ring in a shark cage. The next night on Raw, Cass suffered a legit knee injury, a torn ACL, during a Brooklyn Street Fight against Amore. Cass underwent surgery and was out of action for eight months.
Cass returned during the 2018 Superstar Shake-up, which moved him to SmackDown. On the April 17, 2018 episode of SmackDown, he interrupted the main event and attacked Daniel Bryan. The following week on SmackDown, Cass would take Bryan's spot as the guest on Miz TV, where it would be later revealed that Cass had attacked Bryan backstage. At Greatest Royal Rumble, Cass would enter the 50-man Royal Rumble match at number 49, where he would eliminate Bryan, lasting until the final two before being eliminated by Braun Strowman. At the Backlash pay-per-view, Cass was defeated by Bryan after submitting to the Yes Lock, only for Cass to attack Bryan after the match. On the May 29 episode of SmackDown, he would unsuccessfully wrestle against Bryan and Samoa Joe in a triple threat Money in the Bank qualifying match after Joe applied the Coquina Clutch on Bryan. At Money in the Bank, Cass lost to Bryan again in a singles match via submission. This would be his last match with the WWE, as he was released on June 19, 2018.