Start He made his professional debut for Start on 19 July 2014 in a 2–1 win against
Bodø/Glimt. At 16 years of age, Ajer captained Start in a 1–1 draw against Lillestrøm on 7 April 2015, becoming the youngest ever footballer to captain a team in the
Tippeligaen. While playing for IK Start he played both as a central midfielder, but was gradually placed as a central defender.
Celtic In early January 2016, Ajer had a one-week trial with Scottish club
Celtic. On 17 February 2016, he signed a four-year contract with Celtic and joined the club in June 2016 for a fee which could rise up to £650,000. He was loaned to
Kilmarnock in January 2017 and returned to his parent club to become a mainstay in central defence. On 14 May 2018, Ajer penned a new four-year deal with
Celtic. On 30 August, Ajer scored his first goal for Celtic in a 3–0 win against
FK Sūduva. On 24 October 2019, Ajer made his 100th appearance in all competitions for Celtic's first team in a
Europa League match against
Lazio.
Brentford match in 2025. On 21 July 2021, Ajer completed a move to
Premier League club
Brentford on a five-year deal for an undisclosed fee. He made his Premier League debut on 13 August 2021 in a 2–0 win against
Arsenal. He scored his first goal for Brentford against
Southampton on 7 May 2022. On 21 November 2023, Ajer signed a new deal with Brentford until the summer of 2028, with the club keeping an option to extend his contract for a further season. He scored his second goal against
Burnley on 16 March 2024. ==International career==