At the 2017 Junior World Championships he won a relay gold medal and a skiathlon bronze medal, following up with gold medals in both events at the 2018 Junior World Championships. He made his World Cup debut in December 2018 at Beitostølen, where he also collected his first World Cup point with a 30th place in the 30 kilometres race. At the 2020 Under-23 World Championships in Oberwiesenthal, Amundsen won three individual medals; one of them the gold medal at the 30 km freestyle event.