After turning 17 and getting a British driver's license, Mikkelsen started competing in rallies in the United Kingdom and Ireland.

For the 2008 season, Mikkelsen had some exclusive coaching from Ford's retired double world champion Marcus Grönholm.

[4] He intended to compete in all of the championship rounds in Europe, whilst completing his final year in school.

During Rally Larvik in September 2009, Mikkelsen was involved in an accident, when he got a slide on his car and hit a 10-year-old spectator, killing her instantly.

Mikkelsen drove a Super 2000 car also on three WRC rounds, and won the SWRC class at the season-ending 2010 Wales Rally GB.

[7] Mikkelsen continued his dominance of the IRC in the 2012 season, claiming the drivers championship after winning at Azores and Romania plus collecting five second place finishes.

[8] In early 2017, Mikkelsen entered three rounds of the WRC-2 with a factory Škoda Fabia R5, winning the Monte Carlo Rally and Tour de Corse.

After finishing second in Germany, Mikkelsen signed a deal for 2018 with Hyundai Motorsport, joining close friend Thierry Neuville, Dani Sordo and Hayden Paddon.

[9] Mikkelsen was left without a drive for 2020 season, so at Rally Sardegna he co-drived for 2003 World Drivers' Champion Petter Solberg.

They contested the rally in Shakedown and Power Stage using Saintéloc-prepared Citroen C3 WRC equipped with Pirelli tyres to be used for the 2021 season.

For 2021, Mikkelsen joined Toksport for a combined WRC-2 and ERC campaign, driving a Škoda Fabia R5/Rally2 evo.