Amundsen, who starts for the Asker Skiklubb, took part in junior competitions until 2018. At the Nordic Junior World Ski Championships 2017 in Soldier Hollow , he won the bronze medal in the skiathlon and the gold medal in the relay. He was also fourth there over 10 km freestyle. In January 2018 he ran to third place with the relay at the Norwegian Championships. At the Nordic Junior World Ski Championships in Goms the following month , he won gold medals in the relay and in the skiathlon and came fourth in the classic 10 km run. In the 2018/19 season he finished in the top ten five times in the Scandinavian Cup , including third place over 15 km freestyle in Östersund and thus achieved third place in the overall standings. In December 2018 he started in Beitostølen in the cross-country skiing world cup for the first time and got his first world cup point with 30th place over 30 km freestyle. In the following season he was third in the overall ranking of the Scandinavian Cup. At the U23 World Championships 2020 in Oberwiesenthal , he won the bronze medal in the sprint, the silver medal over 15 km classic and the gold medal with the relay and in the 30 km mass start race.
Norway
