Collier is a front row specialist who cites his interest in the 'dark arts' (of scrummaging) inspired in part by Chris Ritchie, a veteran London Welsh RFC player and Joe Launchbury, a former England Rugby international.[1] His first club was Rosslyn Park F.C. where he began at six years old and played in their mini, youth and First XV squads.[1]
Collier was included in the senior England squad for their 2017 tour of Argentina.[9] On 10 June 2017 he made his Test debut as a substitute against Argentina.[10] A week later Collier scored a try as England won the series.[11] Ultimately these were his only two caps.[2]
In the 2020–21 season, Collier was a replacement in both the Premiership semi-final victory over Bristol Bears and the final against Exeter Chiefs on 26 June 2021, as Harlequins won the game 40–38 in the highest scoring Premiership final ever.[12][13] The following year saw Collier start for an England XV side in an uncapped match against the Barbarians.[14]
In his final season at Harlequins Collier started for the side that were eliminated by Toulouse in the semi-final of the 2024 Champions Cup.[15] After more than a decade with Harlequins making 240 club appearances, Collier joined Castres Olympique for the 2024–25 season.[2]