Nielsen worked as a licensed real estate agent between 1992 and 2021, and he was elected to the Viborg Municipality city council in 2009, where he was re-elected in 2013.

[1] Nielsen succeeded the former mayor, Søren Pape Poulsen, when he was appointed as the new leader of the Conservative People's Party in 2017.

[2] Nielsen is a licensed real estate agent and started running his own business in 1992, first in his hometown Sparkjær, before moving to Stoholm in 2012.

[2] On 3 September 2014, he took over as mayor following the departure of Søren Pape Poulsen, who became the new leader of the Conservative People's Party, while Social Democrat Flemming Lund assumed Nielsen's former position as chairman of the Climate and Environment Committee.

[5] However, Nielsen faced defeat by fellow Venstre candidate Ulrik Wilbek in the contest for the mayor's position in Viborg during the 2017 Danish local elections.