Early career in 2018 Colwill joined
Chelsea at under-nine level, having previously played for Sunday League team City Central alongside
Jamal Musiala. He signed his first professional contract with Chelsea in February 2020 on his 17th birthday. He made his debut on 1 August in a 4–2 penalty shoot-out win against
Sheffield Wednesday in the
2021–22 EFL Cup and his league debut six days later in a 1–1 away draw with
Derby County. On 21 August, he scored his first career goal in Huddersfield's 2–1 win at
Sheffield United on 21 August. Throughout the season he suffered from ankle, knee and hip problems, as well as COVID-19. On 29 May 2022, Colwill scored an
own goal as Huddersfield lost 1–0 to
Nottingham Forest at Wembley in the
2022 Championship play-off final. On 5 August 2022, Colwill joined
Premier League club
Brighton and Hove Albion on loan for the
2022–23 season. He made his debut two days later against
Manchester United, coming on as a stoppage time substitute for
Solly March in Brighton's 2–1 win. Colwill made his first Premier League start in a 2–1 home defeat against
Aston Villa on 13 November. He ended the season having made 13 starts in the
2022–23 Premier League as Brighton qualified for the
UEFA Europa League for the first time in the club's history. On 1 June 2023, it was reported that Chelsea had rejected a £30 million bid from Brighton for Colwill. Colwill complimented Brighton manager
Roberto De Zerbi, and said his own game had room to improve.
Return to Chelsea On 1 August 2023, Colwill signed a new six-year contract with Chelsea, with an option for a further year. He made his official debut for Chelsea on 13 August, playing the full match as Chelsea drew at home to
Liverpool in their opening match of the
2023–24 season. On 3 December, Colwill scored his first goal for Chelsea in a 3–2 victory over Brighton in the
2023–24 Premier League. Three days later, he
captained Chelsea for the first time in a 2–1 away loss to Manchester United. During the first half of the season, he featured frequently as a
left-back. On 25 May 2025, Colwill scored the decisive goal in a 1–0 away victory over
Nottingham Forest on the final matchday of the season, securing Chelsea's fourth-place finish in the league and qualification for the
2025–26 UEFA Champions League. In August 2025, he underwent surgery for an
ACL injury sustained during training sessions after the
FIFA Club World Cup triumph, an injury which would sideline him for most of the 2025–26 season. ==International career==