Liljegren made his Swedish Hockey League debut on 3 December 2015 playing with Rögle BK during the 2015–16 SHL season.

[2] On 23 June 2017, Liljegren was selected in the first-round, 17th overall, by the Toronto Maple Leafs in the 2017 NHL entry draft.

[3] Liljegren was assigned to the Maple Leafs' American Hockey League (AHL) affiliate, the Toronto Marlies, for their 2017–18 season.

Liljegren made his NHL debut with the Maple Leafs on 18 January 2020 in a 6–2 defeat against the Chicago Blackhawks.

[6] Liljegren scored his first National Hockey League goal on 15 January 2022 against Jordan Binnington of the St. Louis Blues.