After a loan spell at Fulham, he remained in London to sign permanently with Crystal Palace in 2021, where he made 112 appearances over three years.

At Midtjylland he moved into the talent academy in Ikast[4] where players like Simon Kjær, Winston Reid, Erik Sviatchenko, Viktor Fischer and Pione Sisto had stayed before him.

[citation needed] In April 2013, before he had made his first team debut with Midtjylland, Twente sent out scouts to observe Andersen, and offered him a one-week trial, which he accepted.

[citation needed] Though he was still an under-19 player he was promoted to the senior team and on 8 November 2013, Andersen made his first appearance for Jong Twente playing the whole match.

[16] On 19 December, Andersen was sent off in a 1–1 draw with Newcastle United, receiving a second yellow when he gave away a controversial penalty for a foul on Callum Wilson which was dispatched by the latter.

[17] Andersen was nominated for the league's Player of the Month award for February 2021 after a number of impressive defensive displays that saw Fulham concede just three goals in six matches as they picked up a vital nine points in their fight against relegation.

[20][21] In his first season, he made 39 appearances, forming a centre-back partnership with Marc Guéhi and starting in Palace's run to the FA Cup semi-final.

[25] On 23 August 2024, Andersen returned to Fulham, this time on a permanent basis, signing a five-year deal with the club for a reported fee of £30 million.

[30] In the round of 16 match against Germany, he had a goal disallowed for offside and later conceded a penalty for handling a cross, as the game concluded with a 2–0 defeat.

In the 2018–19 season, Inter Milan midfielder Marcelo Brozović was the only player in Serie A with more accurate long passes than Andersen.