On 7 October 2020, Nielsen made his debut for Hvidovre's first team, playing all 90 minutes in a cup match against KFUMs Boldklub København.
[3] His good form also attracted interest in the summer window, but Hvidovre did not want to sell the young left-back.
With 21 appearances in the 2022–23 season, Nielsen played an important role in Hvidovre's promotion to the 2023–24 Danish Superliga.
[5] Nielsen made his debut in the top Danish league on 21 July 2023, against FC Midtjylland.
[6] On July 9, 2024, newly promoted Danish Superliga side AaB confirmed that they had bought Nielsen free in Hvidovre, but only from January 2025.