Mikkelsen played youth football for Bredebro IF before joining his first senior club, FC Sydvest 05, where he scored 11 goals in 14 league appearances.

[2] He scored the winning goal for Dundee United after coming on as a substitute in the 2017 Scottish Challenge Cup Final, as they won 2–1 against St Mirren.

[3] On 4 July 2017, it was announced that Mikkelsen had signed a two-year deal with Scottish Premiership club Ross County.

After just two league starts and seven appearances from the bench, Mikkelsen returned on loan to Dundee United in January 2018 for the remainder of the 2017–18 season.

[5] Mikkelsen along with five other first-team players were released from Ross County after the club's relegation to the Scottish Championship.