He made his debut for Silkeborg IF's first team in the Danish Superliga on 25 August 2017 against Hobro IK.
In March 2019, Mattsson suffered a long-term injury in his groin, which kept him on the sidelines for half a year.
Unluckily, Mattsson suffered an injury to his left anterior cruciate ligament in that game and was again out for a longer period.
[5] Mattsson joined Copenhagen on 1 February 2024 signing a contract with the Danish side through the 2027–28 campaign.
[6] He debuted for the club on 13 February 2024 by scoring in the first leg of the Champions League Round of 16 against Manchester City in a 3–1 loss.