Big Cass's WWE NXT television debut took place on 5 June 2013, under the ring name Colin Cassady with a loss to Mason Ryan. Cassady then formed an alliance with Enzo Amore, who had also previously lost to Ryan, and they labelled themselves 'the realest guys in the room'. However, Ryan easily defeated them in consecutive singles matches, but lost to them in a handicap match. With their feud with Ryan finished, Amore and Cassady went on to feud with Alexander Rusev, Sylvester Lefort and Scott Dawson. When Amore suffered a broken leg, Cassady tried his hand at singles competition, feuding with Aiden English. Amore returned on the 26 June 2014 episode of NXT, saving Cassady from an attack from Sylvester Lefort and Marcus Louis. In early August, Cassady and Amore participated in the NXT Tag Team Championship tournament, where they defeated Tye Dillinger and Jason Jordan in the first round but lost to 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. As a result, Amore challenged Lefort to a match with the loser's hair as forfeit at NXT Takeover: Fatal 4-Way on 11 September. Amore proceeded to win 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. 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 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 in a number one contender's match for a title opportunity against Blake and Murphy. Amore and Cassady received their title match at TakeOver: Unstoppable, which they would lose after Alexa Bliss interfered. At NXT TakeOver: London, Enzo and Cass challenged Dash Wilder and Scott Dawson for the NXT Tag Team Championship in a losing effort. They were granted another title opportunity against Dawson and Wilder on March 19, 2016 at WWE Roadblock, but were once again unsuccessful.
On the April 4, 2016 episode of Raw (which took place the night after WrestleMania 32), Amore and Cassady made their main roster debuts and confronted The Dudley Boyz. On the April 14 SmackDown, they made their main roster in-ring debuts, defeating The Ascension in the first round of the WWE Tag Team Championship Number One Contender's Tournament. In the finals, Amore & Cassady faced The Vaudevillains at Payback, with the match ending in a no contest after Amore suffered a legitimate concussion during the match. Cassady's ring name was then changed officially to Big Cass on the May 9 Raw. On the June 6th edition of Raw, Enzo and Cass battled the Vaudevillains. During the match, Cass witnessed English attempting to injure Amore in a similar manner of what he did to him at Payback, which led to Cass attacking the Vaudevillains, including giving an East River Crossing to Simon Gotch. On the July 19 Smackdown Live, Big Cass and his tag team partner Enzo Amore were chosen by the General Manager of Monday Night Raw Mick Foley and Raw commissioner Stephanie McMahon. Amore and Cass were drafted in round 4. Following Battleground, Enzo and Cass proceeded to feud with Chris Jericho and Kevin Owens, now collectively known as "Jeri-KO". The feud culminated at SummerSlam in a tag team match, which would ultimately be won by Jericho and Owens. On the August 22 episode of Raw, Cass qualified for the Fatal 4-Way match for the WWE Universal Championship, by beating Rusev in a one-on-one matchup by countout. The next week on Raw, he went on to face Seth Rollins, Roman Reigns and Kevin Owens, where he was eliminated by Owens, marking the first time he was pinned on the main roster. Enzo and Cass then started a feud with Primo & Epico, losing to them in tag team and singles matches throughout September.
On the November 7 episode of Raw, Enzo and Cass were announced as part of Team Raw for the 10–on–10 Survivor Series Tag Team Elimination match at Survivor Series on November 20, 2016, which Team Raw won.