Career
He will join Scottish club Celtic in June 2016. Ajer was born in Norway and he started his career with Rælingen FK before joining Lillestrøm when he was 10 years old. At 16 years of age, Ajer captained Start in a 1–1 draw against Lillestrøm on 7 April 2015, becoming the youngest ever football player to captain a team in the Tippeligaen.
In early January 2016, Ajer had a one week trial with Scottish club Celtic.
On 17 February 2016 Ajer signed a four-year contract with Celtic and will join the club in June 2016.