Nielsen transferred from the Belgian club Union SG to Club Brugge in the summer of 2022. The Danish midfielder signed his first professional contract at the age of fifteen and made his debut for the first team in 2012. In January 2017, he moved to Danish club Odense BK, where he quickly became a key player.
Two and a half years later, Nielsen transferred to the Belgian second division club Union Saint-Gilloise. Once again, he became an integral part of the team. In 2021, Nielsen helped Union achieve the promotion to the top division. They came close to winning the championship the year after, narrowly missing out. In the Champions' Play-Offs, FCB managed to surpass Union.
Nielsen's excellent season was rewarded with an offer from Club Brugge. The Danish midfielder gladly accepted and signed a contract until 2026.