Born in Sanderum, a suburb of Odense, Thomas started playing football in the youth of Sanderum Boldklub before moving to Odense Boldklub's youth academy at age 11.

[1] On 24 April 2015, Thomasen made his debut for OB in a 2–0 win over Esbjerg fB in the Danish Superliga.

In the summer of 2015, at the age of 19, Thomasen was moved permanently to the first team squad.

[6] He made his official debut for the club on 30 July 2022 in a 1–0 defeat against Caen in Ligue 2.

[7] A year after his transfer to France, it was confirmed, in July 2023, that Thomasen had joined Swedish Allsvenskan club IF Elfsborg on a deal until the end of 2025.