After helping Lyngby BK gain promotion to the Danish Superliga, Mikkelsen switched to league rivals Brøndby IF in the summer of 2007.

On 21 July 2007, he made an impressive home debut for Brøndby in their match against FC Nordsjælland scoring a goal and also notching an assist a 2–2 draw.

On 30 July 2015, he scored a brace in a 3–2 win over Hungarian side Debreceni in the first leg of the Europa League third qualifying round.

[citation needed] In May 2018 Mikkelsen was linked with a move to Australian club Brisbane Roar[2] and subsequently signed a one-year contract in June.

[3] Mikkelsen made his Roar debut in the FFA Cup in August 2018 at home to Melbourne City Mikkelsen received his first international call up to the Danish national team in November 2011, to face Sweden and Finland, making his debut for Denmark against Finland 15 November 2011.