Getting his football education in the youth ranks of local team Esbjerg fB, he saw limited first team opportunities, and in the summer of 2010 he decided to join Vejle Boldklub in the
Danish 1st Division. After two successful seasons for Vejle, both of them in the Danish 1st division, bigger teams started to pay him attention. In March 2012 he was sold to Norwegian club Rosenborg BK. He became a regular first team starter for the Norwegian side, but after only 18 games he had yet to settle personally. In the same summer he was sold back to his childhood club Esbjerg fB on a four-year contract. During his second stay at Esbjerg, alongside twin brother Jakob Ankersen, played an integral part of the Esbjerg team that following promotion won the
Danish Cup and finished 4th in the
Danish Superliga. During a 5–1 win over
AGF Aarhus on 10 August 2013, Ankersen scored a sensational goal reminiscent of
Marco van Basten's strike in the
final of Euro 88. The goal was shortlisted for the 2013
FIFA Puskás Award. In January 2014 he was signed by
RB Salzburg after impressing a against them in the group stages of the Europa league, and was praised for his ability to play on either defensive flank. He joined F.C. Copenhagen in the summer of 2015 initially on loan, before signing permanently the season after. He was linked with a move to the
Premier League club
Newcastle United. Ankersen was nominated for the goal of the season by the Danish football federation in 2017. On transfer deadline day, 2 September 2019, Ankersen joined Italian
Serie A club
Genoa on a contract until June 2021 with an option for a further year. He made 19 league appearances during his sole season with Genoa, where he was mostly used as a substitute. On 26 September 2020, FC Copenhagen announced that they had signed Ankersen on a four-year contract. On 1 October 2020, Ankersen scored a highly bizarre own goal against Croatian club
HNK Rijeka in the
2020-21 UEFA Europa League play-off round. The own goal proved to be decisive and FCK was eliminated. After four years in Copenhagen, it was confirmed on 31 March 2024 that Ankersen would be leaving the capital club at the end of the season. On 1 June 2024
FC Nordsjælland confirmed that they had signed Ankersen on a free transfer, where Ankersen signed a contract until June 2026. ==International career==