He played for Melbourne on the wing in their 2009 NRL Grand Final win against the Parramatta Eels (which was stripped from the club after their salary cap scandal).

Nielsen was named as injured Greg Inglis' replacement at centre in the Queensland side for Game I and II of the 2011 State of Origin series.

It was alleged that a woman had made a complaint to the St George hierarchy claiming that an intoxicated Nielsen bit her on the leg on a night out on 22 March 2015.

[12][13] After several games in the Mackay & District Rugby League, Nielsen signed a year and a half contract with the Bradford Bulls on 28 July 2015.

[17] He played most of the first half of the season for the North Sydney Bears with the likes of Alex Johnston, Bryson Goodwin, Aaron Gray, Hymel Hunt, Robert Jennings, Siosifa Talakai, Cody Walker and even backrower Tyrell Fuimaono obtaining positions in the centres and on the wings.

On 21 July 2017, English side Featherstone Rovers offered Nielsen a contract which he agreed to in principle but the deal fell through due to visa issues.