Esbjerg Nielsen signed his first professional contract with
Esbjerg in June 2009 only 15 years old. A few months later, he went on a trial with English
Premier League club
Manchester City. His contract was prolonged in January 2013, where he officially was moved up into the Esbjerg first team squad. Later in the season, he made his professional debut on 12 May 2013 in a 2–0 win against
Copenhagen.
Odense In January 2017, Nielsen signed with
Danish Superliga team
Odense BK on a -year contract.
Union SG On 4 July 2019, Nielsen joined Belgian club
Union SG.
Club Brugge On 17 July 2022,
Club Brugge announced that they had reached an agreement to transfer Nielsen from Union SG. Nielsen signed a four-year contract to keep him with Club through 2026. He scored his first goal for Club on 14 August 2022, the winner in a 3–0 victory over
OH Leuven.
Standard Liège On 2 July 2025, Nielsen signed a three-year contract with
Standard Liège. ==International career==