In June 2013, Dempsey signed a contract with WWE. He was assigned to the WWE Performance Center. Dempsey made his debut in NXT on September 12, 2013, where he was defeated by Aiden English. Throughout late 2013, he made sporadic appearances as an enhancement talent. In the late spring to summer, Dempsey returned to NXT as a tweener, squashing various low carders and local talents. In August, Dempsey and Mojo Rawley formed an uneasy tag team and participated in the NXT Tag Team Championship tournament, where they were defeated by The Vaudevillains (Aiden English and Simon Gotch) in the first round, prompting Dempsey to attack Rawley in the aftermath and establishing himself a heel. On September 11 at the NXT TakeOver: Fatal 4-Way event, Dempsey defeated Rawley in a squash with his new finisher The Diving Headbutt. On the September 25 episode of NXT, Bull was victorious in the rematch. Following that, he embarked on a rivalry with Baron Corbin, another dominant wrestler who mostly squashed jobbers. The two competed to see how quickly they could defeat the "local talent", sometimes back-to-back. This eventually led to a match between the two on January 14, 2015, in which Dempsey's undefeated streak was broken by Corbin.
Dempsey competed in the Andre the Giant Memorial Battle Royal Qualification Tournament at WrestleMania 31 Axxess, but lost to Tyler Breeze in the first round. Soon after this, Dempsey's gimmick was tweaked to that of an overweight, out of shape wrestler who eats too much, which also saw him turn face in the process. After several vignettes aired to promote "Bull-Fit", Dempsey debuted his new gimmick on the August 26 episode of NXT, where he defeated Elias Samson. On the February 24 episode of NXT, he was defeated by Tommaso Ciampa. On the March 23 episode of NXT, he and his supposed opponent Danny Burch both were beaten down by Samoa Joe, before they even started the match, this led to a match between Dempsey and Joe on the following weeks episode. Dempsey lost in a quick squash match by disqualification because Joe kept his finisher, the Coquina Clutch, on Dempsey prompting NXT General Manager William Regal to try to stop him. Samoa Joe's insubordination would bring NXT Champion Finn Bálor to the ring to "rescue" Dempsey. The following week Dempsey lost to Joe in a squash match, this was Dempsey's final appearance for NXT.
On February 5, 2016, Dempsey was released by WWE.