Born in Nykøbing Falster, Michelsen took up speedway at the age of 6, and went on to win the Junior World Cup in both 2008 and 2009.

After racing in Germany for AC Landshut Devils in 2012, he rode in Poland and the United Kingdom in 2013 for Unia Leszno and Eastbourne Eagles respectively, staying with both teams in 2014.

[3][1] In 2014 he was a reserve in the Danish Speedway Grand Prix and finished third in the World Under-21 Championship after also reaching the final in 2011 and 2013.

[10] He was also part of the Danish team that won the bronze medal in the 2023 Speedway World Cup final.

[13] His 2024 season ended badly after a crash with Bartosz Zmarzlik at the Latvian Grand Prix, where he sustained significant injuries to his right shoulder, humerus and elbow.