Mikkelsen played youth football for Tromsdalen UIL and Tromsø IL,[2] and attended the Norwegian School of Elite Sport in Tromsø. He started his senior career in Tromsdalen, on loan in the summer and fall of 2018.[3] In 2019, he was back in Tromsø and made his Eliteserien debut in May 2019 against Odd. He was also capped for Norway U19. In the 2020 1. divisjon he played semi-regularly for Tromsø, that won a promotion back to the top tier.[2]
In 2021, Mikkelsen had his major breakthrough, scoring eight goals in 27 league appearances, as the newly promoted side finished 12th in the table. On 9 December, after the last league fixture, Mikkelsen signed a new four-year contract with the club. He was one of three players nominated to win Eliteserien Young Player of the Year, but the award was eventually given to Mads Hedenstad Christiansen.[4] Mikkelsen retained his fine form in 2022, scoring seven goals and providing five assists in 26 appearances, helping his side to finish 7th in the Eliteserien table. After the season, Mikkelsen was voted Tromsø Player of the Year by the supporters of the club.[5]
Hammarby
On 25 January 2023, Mikkelsen transferred to Hammarby IF in Allsvenskan, signing a five-year contract.[6][7] The transfer fee was reportedly set at around €2 Million, with potential bonuses included.[8] Throughout his debut season in Sweden, Mikkelsen struggled to make an impression, making 21 goalless league appearances.[9][10][11]
Bodø/Glimt
On 24 March 2024, Mikkelsen moved to Bodø/Glimt signing a four-year contract.[12]