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 worked in the same fish factory as Jonas Vingegaard while pursuing a career as a pro cyclist.[9]
He has ridden the Tour de France 6 times (2015 to 2020) as well as the 2014 Vuelta a Espana. 2018 served as a breakout year seeing him win the classics Omloop Het Nieuwsblad and the Amstel Gold Race.[11]
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.[14] He was unable to race for the rest of the year, and moved down to the EF Education–Nippo Development Team the following year as he transitioned back to racing. He returned to competition on April 30, 2023 and soon after announced he would rejoin the World Tour squad the following year.[15]