[4] Eventually he started competing in the sport, quickly taking his first amateur mixed martial arts bout.
He amassed a record of 8–1 before signing with the UFC on the heels of a first round rear naked choke finish of Gabriel Checco at RFA 19.
[5] Collier made his promotional debut against Vitor Miranda on December 20, 2014 at UFC Fight Night 58.
[7] Collier next faced Ricardo Abreu on June 6, 2015 at UFC Fight Night 68, filling in for an injured Daniel Sarafian.
[9] Collier faced returning veteran Yang Dongi on November 28, 2015 at UFC Fight Night 79.
[13][14] Collier was expected to face Josh Stansbury in a light heavyweight bout on December 3, 2016 at The Ultimate Fighter 24 Finale.
[15] However, Collier pulled out of the fight in late October citing injury and was replaced by Devin Clark.
[35] As the first bout of his new four-fight contract, Collier was scheduled to face Justin Tafa on April 30, 2022, at UFC on ESPN 35.
[47] However, Walker withdrew from the bout due to unknown reasons and was replaced by Mohammed Usman.
[50] Collier faced Kelvin Fitial at Shamrock FC 357 on September 7, 2024 and won the bout by knockout in the first round.