Colwill joined the youth academy of Cardiff City Football Club when he was eight.[2] He made his professional debut with the club as a late substitute during a 3–1 EFL Championship win over Coventry City on 13 February 2021.[3] The following day, Colwill signed a new two-year contract with the side.[4] Colwill's first league start for the club came in a 2-1 win over Wycombe Wanderers on 24 April 2021, earning high praise from manager Mick McCarthy for his performance in the match.