[1] He played as a junior for Newcastle Basketball Association[1] and attended St Marys Senior High School.

[16] Nielsen won the gold medal at the 1997 FIBA Under-21 World Championship with Australia's junior national team.

[19][20] In 2014, Nielsen joined the San Antonio Spurs coaching staff in a player development role, on a contract that ran through to the end of the 2015 NBA Summer League.

[21][22] 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.