Youth Andersen started playing football at the age of four for local club
Greve Fodbold, where he played from 2000 until 2009. He then joined
Copenhagen's academy, School of Excellence, before leaving for
Midtjylland when he was 15. At Midtjylland he moved into the talent academy in
Ikast where players like
Simon Kjær,
Winston Reid,
Erik Sviatchenko,
Viktor Fischer and
Pione Sisto had stayed before him. 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. Four months later, he signed a youth contract with FC Twente for a fee rumoured to be around 5 million Danish kroner (€650,000). The plan was to start in the youth team to acclimatise, and progress to the senior sides after about 6 months.
Twente 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. He did not play any matches for the first team in his first season, but already in his second year at the club he was promoted to train with the first team squad with players like
Dušan Tadić,
Hakim Ziyech,
Jesús Manuel Corona,
Quincy Promes,
Andreas Bjelland,
Kasper Kusk and
Kamohelo Mokotjo. On 7 March 2015, he got his chance in the first team debuting against
Willem II, and playing the last 20 minutes. A week later, he signed a new contract with Twente until 2018. In 2015, Andersen won the prize for U19 talent of the year by the
Danish Football Association. On 22 March, he was named in the starting eleven against
Groningen and scored his first goal for the first team. He slowly got more and more time on the pitch for the first team. He continued his development and became almost a regular part of the first team squad in the 2015–16 season.
Sampdoria On 26 August 2017, Andersen signed a contract with
Serie A club
Sampdoria. He had offers from other clubs in different leagues but chose Sampdoria to develop in Serie A. Andersen made his debut on 25 February 2018, in a match against
Udinese. He played seven more matches that season and from the beginning of the 2018–19 season he became a regular starter attracting attention from bigger clubs. In his second season he only missed one game due to a ban. Sampdoria offered him a new contract until summer 2022 which he signed on 8 November 2018.
Lyon in 2019 On 12 July 2019, Andersen signed a five-year contract with French side
Olympique Lyonnais. The transfer fee amounted to €30 million (24 + 6 million in bonuses), meaning Andersen became Lyon's record transfer. It was also a record high transfer fee for a Danish football player. On 2 October, he made his debut in the
Champions League against
RB Leipzig, and on 11 November he scored his first Champions League goal in the 3–1 win against
Benfica.
Loan to Fulham On 5 October 2020, Andersen joined
Premier League club
Fulham on a season-long loan. 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. 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. On 19 March, Andersen scored his first Fulham goal in a 1–2 home league defeat to
Leeds United.
Crystal Palace On 28 July 2021,
Crystal Palace announced the signing of Andersen on a five-year deal for a fee of €17.5 million with €2.5 million add-on bonuses and a 12.5% sell on fee. 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. In
the following season, Andersen was headbutted by
Darwin Núñez, for which the Uruguayan was sent off on his home debut for
Liverpool, a 1–1 draw with Palace on 15 August 2022. On 27 August, Andersen scored his first goal for the club to put Palace 2–0 up away at champions
Manchester City, though City ultimately won 4–2. Andersen's second goal for Palace was a late equaliser at
Brentford on 26 August 2023, almost a year after his previous goal. He later scored the only goal, described as a "superb strike", in a victory over
Manchester United at
Old Trafford on 30 September. He finished the
2023–24 season as the player with most clearances in the Premier League in 229 occasions.
Return to Fulham 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. == International career==