In 2015, he signed with Norwegian Tippeligaen club Sarpsborg 08, where he was part of their successful UEFA Europa League campaign in the 2018–19 season, reaching the group stage.
[1][2] He officially made his debut as a 17-year-old for the club's first team on 15 October 2006, a month after his 17th birthday, in a home game at Sundby Idrætspark against Lyngby Boldklub in the 1st Division, when he came on as a substitute for Lars Brøgger in the 84th minute.
[3] Mortensen subsequently reached a total of 10 appearances with one goal, in the home game against Thisted on 15 April 2007, in the 2006–07 season, after which Fremad Amager suffered relegation to the third-tier Danish 2nd Division.
[4][5][6] Mortensen began his tenure at the club's reserve team in the third-tier Danish 2nd Division East, where he made his debut on 19 August 2007 in an away match against FC Roskilde.
[5] In early March 2008, Mortensen was on a three-day trial practice with the northern French Ligue 1 club Lille, but this did not result in a move.
On 29 August 2010, Mortensen managed to score his first goal in the top division, in the 3–3 home draw against his former team Brøndby.
[11] Lyngby suffered relegation back to the 1st Division at the end of the next season, with Mortensen remaining in the team until the summer of 2015.
[16][17] With Sarpsborg, he participated in the group stage of the 2018–19 UEFA Europa League where he distinguished himself on 4 October 2018 by scoring a brace against Belgian club Genk, contributing to the 3–1 victory of his team.
In November 2020, he was called up to Kasper Hjulmand's senior squad for the friendly against Sweden due to several cancellations from, among others, the Danish national team players playing in England, due to the COVID-19 restrictions, as well as a case of COVID-19 in the squad, which had put several national team players in quarantine.