In August 2021, Julius and his brother Brutus Creed appeared on WWE NXT as members of Roderick Strong's Diamond Mine stable. They made their in-ring debut at the August 24, 2021 WWE NXT tapings (with their match airing on September 7, 2021), defeating Chuckie Viola and Paxton Averill in a squash.
On February 15, 2022, at NXT Vengeance Day, The Creed Brothers defeated MSK to win the men's Dusty Rhodes Tag Team Classic. On June 4, at NXT In Your House, they defeated Pretty Deadly to win the NXT Tag Team Championship. At Worlds Collide on September 4, The Creed Brothers lost the NXT Tag Team Championships to Pretty Deadly in a fatal-four way elimination tag team title unification match, involving the NXT UK Tag Team Champions Josh Briggs and Brooks Jensen, and Gallus (Mark Coffey and Wolfgang), after Damon Kemp turned on Diamond Mine by hitting Julius Creed with a steel chair, ending their reign at 92 days.
On the July 4, 2023 edition of NXT, the Creed Brothers faced the Dyad in a "Loser Leaves NXT" tag match, but were unsuccessful, forcing them to leave the brand. They were, however, reinstated the following month after defeating the Dyad in a cage match.
The Creed Brothers, along with fellow Diamond Mine member Ivy Nile, made their main roster debut on the October 30 episode of Raw, defeating Alpha Academy (Chad Gable and Otis). On the November 27 episode of Raw, The Creed Brothers won a tag team turmoil match for an Undisputed WWE Tag Team Championship opportunity. The brothers would fail to defeat The Judgment Day (Finn Bálor and Damian Priest) for the titles on the December 18 episode of Raw. In July 2024, the Creed Brothers aligned themselves with Chad Gable by assisting Gable to attack Bo Dallas, turning heel for the first time since 2022 and the first time on the main roster. The stable of Gable and the Creed Brothers was then subsequently dubbed "American Made."