Career
He plays at centre back for Air Corps Horsens in the Danish First Division. Nielsen has 1 youth cap for Denmark, playing for the Danish under-20s. A versatile player, Nielsen"s preference is playing as a defensive midfielder, though mostly utilized at centre back and occasionally right back.
Mathias Nielsen spent his entire youth career with Herfølge Boldklub, joining the club at 4 years of age, he went on to sign a contract with this childhood club in the summer of 2007.
During the winter he would play indoor football with the team"s indoor division, playing as goalkeeper. When Herfølge BK merged with Køge BK, Nielsen was carried over in to the newly formed club HB Køge.
F.C. Nordsjælland
1 July 2010 Nielsen moved to Superliga team F.C. Nordsjælland on a free transfer, signing a 1 year contract. He made his debut the following month as a substitute during the second leg of FCN"s third qualifying round of the Europa League, against Portuguese side Sporting Lisbon.
Amid interest from Italian clubs Parma and Napoli in March 2011, the young defender extended his contract with FCN for another two years.
17 July 2012, Nielsen left FCN for Næstved BK, having made a single appearance for the Farum club