In 1995, Nielsen played for the Sydney Sky in the SEABL[3] and made his NBL debut with the Sydney Kings as a development player.[2][4] In 1996, he moved to Canberra to attend the Australian Institute of Sport (AIS).[5] He played for the AIS in the SEABL.[6] He was named SEABL East Youth Player of the Year in 1995 and 1996.[7] He played six games for the Sky in 1997[8] and played for the Penrith Panthers in the SEABL in 2000.[9]
In 244 games for the Kings over nine seasons, Nielsen averaged 17.5 points, 8.0 rebounds, 2.6 assists, 1.0 steals and 1.5 blocks per game.[13] In 2013, he was named in the Sydney Kings 25th Anniversary Team.[14]
On 29 July 2015, Nielsen returned to the Perth Wildcats, signing with the club as an assistant coach ahead of the 2015–16 NBL season.[23] On 10 April 2019, after three championships in four seasons, Nielsen parted ways with the Wildcats in order to pursue coaching opportunities in the United States.[24]
On 5 November 2019, Nielsen was appointed assistant coach of the Austin Spurs of the NBA G League.[25] On 10 November 2020, he was promoted to head coach of the Spurs.[26]
On 8 September 2021, Nielsen was appointed assistant coach of the San Antonio Spurs.[27]