From the North Queensland town of Mackay, Nielsen is of Indigenous Australian and Solomon Island descent and was a Norths Devils junior.
Nielsen started his NRL career with the Cronulla-Sutherland Sharks, making his début on 20 July 2007 against the Manly-Warringah Sea Eagles.
In 2008 Nielsen joined the Melbourne Storm, becoming a regular at centre. In 2009 Nielsen was sent-off for a high tackle in the Storm's Round 15 clash with the Wests Tigers. 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 then went on to play in Melbourne's 2010 World Club Challenge victory against English champions, the Leeds Rhinos.
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.
In May 2012, he signed a three-year deal with the New Zealand Warriors starting in 2013. Later that year he played in the third and deciding game of the 2012 State of Origin series which Queensland won. At the end of the season Nielsen played for the Melbourne Storm in their 2012 NRL Grand final victory over the Canterbury-Bankstown Bulldogs.
Nielsen moved to Auckland for the 2013 New Zealand Warriors season, also the first for the club's new Australian coach Matthew Elliott.
On 19 November 2014, Nielsen signed a two-year deal with the St George Illawarra Dragons, starting from the 2015 season.
On 5 May 2015, Nielsen had his contract with the Dragons terminated due to disciplinary reasons.
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.
Nielsen featured in Qualifier 3 (Salford Red Devils) to Qualifier 6 (Leigh Centurions). Nielsen played in the £1 Million Game against Wakefield Trinity Wildcats.
On 25 January 2016, Nielsen returned to Australia, signing a two year contract with the South Sydney Rabbitohs. He also played in the NSW Cup for the Rabbitohs feeder club, the North Sydney Bears. Nielsen made his debut for the club in Round 8, playing in the centres in the 30-8 win against the Brisbane Broncos.
Nielsen retired from professional football during the 2017 NRL season. In January 2017, he was sent home from Rabbitohs training after turning up “looking worse for wear”, after an alleged drunken night out. 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.. On 23 July 2017, Nielsen signed on to play with the Mackay Magpies, returning to club that he played with in 2015.