The Norwegian moved to Great Britain in 2006 to be able to take his driving test at the age of 17 – a year sooner than at home. Sixteen days after his 17th birthday, he won the Quinton Stages Rally in Wales, becoming the youngest ever winner of a British national rally.
Mikkelsen made his FIA World Rally Championship debut the same year and went on to finish in the top ten at three of the eight rounds he contested in 2007.
Financial constraints meant Mikkelsen focused on rallies closer to home in 2009, but he returned to the international stage in 2010, driving a Ford Fiesta S2000 in the Intercontinental Rally Championship (IRC).
Impressive pace in the Fiesta brought him to the attention of Škoda UK, which offered him a works-supported drive in the 2011 IRC. It was a breakthrough season, with Mikkelsen claiming the title aboard a Fabia S2000 - a feat he went on to repeat the following year. At the same time, Mikkelsen contested eight rounds of the WRC in a Volkswagen Motorsport-entered Fabia.
Mikkelsen joined the all-new Volkswagen Motorsport team full-time in 2013, contesting the full WRC season in a Polo R WRC. Fourth place at the Acropolis Rally was the highlight of an experience-gaining year.
2014 was one of Mikkelsen's best seasons to date, as he finished third in the WRC. But it was in 2015 that he scored his first WRC win, topping the rostrum in Spain. Two further wins followed in 2016 before Volkswagen departed the championship, leaving Mikkelsen without a drive for 2017.
He and then co-driver Andreas Jæger started the year in WRC2, the premier support category, with Škoda Motorsport. Citroën Racing offered the Norwegian duo three drives in a C3 World Rally Car, with which they scored a P2 finish in Germany before Mikkelsen signed a two-year deal to join Hyundai Motorsport.
He was a regular podium finisher during his stint with the Korean marque, although he failed to win a rally aboard the i20 WRC. When Mikkelsen's contract ran out at the end of 2019, he was again left without a drive.
Determined to get back to the top of the sport, Mikkelsen spent the following three seasons in WRC2 with Škoda. He won the 2021 and 2023 titles, duly earning himself a return to the top level with Hyundai on a part-time basis for 2024.