Milton Keynes Dons Brittain joined
Milton Keynes Dons' academy in 2006 aged 8. On 27 April 2016, Brittain signed professional terms with the club ahead of the
2016–17 season. On 2 May 2016, Brittain joined Icelandic
Úrvalsdeild karla side
Þróttur Reykjavík on a short-term loan deal until 18 July 2016. In total, Brittain featured 8 times for the club including being named
man of the match in several performances. On 28 January 2017, Brittain made his first team league debut for MK Dons, coming on as a late substitute in a 0–4 away win to rivals
Peterborough United. On 14 June 2017, Brittain's contract was extended until summer 2018. On 2 September 2017, Brittain scored his first league goal for the club in a 1–1 draw with
Oxford United having come on as a 35th-minute substitute for
Ethan Ebanks-Landell. Following his first goal for the club and an international call up, Brittain was named
EFL Young Player of the Month for September 2017. In January 2018, Brittain signed a new contract keeping him at the club until June 2020. Having been released at the expiry of his contract due to the economic situation on 30 June 2020, he re-signed on a two-year deal with
Milton Keynes Dons on 25 August 2020.
Barnsley On 10 October 2020, Brittain joined
Championship club
Barnsley for an undisclosed fee, signing a three-year contract. In his debut season at
Oakwell, Brittain started 40 league games for Barnsley. The club reached the Championship play-offs after finishing in fifth-place in the 2020–21 season. They were knocked out by
Swansea City at the semi-final stage of the play-offs. Barnsley were relegated from the Championship in the 2021–22 season.
Blackburn Rovers On 21 July 2022, Brittain joined
Blackburn Rovers for an undisclosed fee on a four-year contract. In July 2025, Blackburn manager
Valerien Ismael said that he wanted to keep Brittain, but the defender had rejected the club's offer of a new contract and had refused to take part in Blackburn's friendly match against
Accrington Stanley.
Middlesbrough On 4 August 2025, Brittain signed for
Middlesbrough for an undisclosed fee. ==International career==