After leaving school in 1985 Nielsen trained as a police officer but resigned after 11 months. He then worked as a truck driver for a Formula One catering company.[2] In 1986, he joined Team Lotus' test team,[2] before running their spare parts department. In 1991, he moved to Tyrrell in the same role before being appointed assistant team manager in 1994.[2] In the next five years he moved to Benetton, back to Tyrrell, to Honda, then Arrows, before moving back to Benetton, where he spent almost a decade as Sporting Director, including the team's transitions to Renault F1 and Lotus Renault GP.[2]
After spending 2012 as Sporting Director at Caterham, Nielsen joined Scuderia Toro Rosso at the start of the 2013 season.[3] He joined Williams as Sporting Manager in December 2014[4][5] and worked in that role until leaving in July 2017, being replaced by Dave Redding.[6]
On 1 August 2017 he started in a new position as the Sporting Director of Formula One.[7] He left this role and became the FIA Sporting Director on 18 January 2023.[8] In December 2023, Nielsen left the FIA after 11 months in the position.[9]