[7][8] He became a professional in 2011 at the age of just 19 riding for the Danish continental team Glud & Marstrand–LRØ, where he won two editions of the U23 Liège–Bastogne–Liège and several other noticeable wins.

He made an immediate impact, as he won the Danish National Road Race Championships and the Danmark Rundt.

2018 served as a breakout year seeing him win the classics Omloop Het Nieuwsblad and the Amstel Gold Race.

[13] At the Route d'Occitanie in June 2022, Valgren was badly injured in a crash, sustaining a fractured pelvis, dislocated hip, injuries to the ACL and MCL and a damaged meniscus.

He returned to competition on April 30, 2023 and soon after announced he would rejoin the World Tour squad the following year.