Denmark Andersen began his career with Danish club
AB and became the youngest goalkeeper to play in Danish senior football. He joined Reading on a two-and-a-half-year contract on 26 January 2007, having played seven times for AB.
Reading Andersen signed a loan deal with
Torquay United on 26 February 2008, making his debut the same day in a 1–1 draw at home to
Altrincham. He returned to Reading having played three times for Torquay during his one-month on loan and looked set to remain with Reading for the remainder of the season. However, he joined
Rushden & Diamonds on a seven-day emergency loan on 15 April 2008, making his Rushden debut in a 2–1 win at home to Torquay. He also played in the defeats against
Exeter City and
Oxford United before returning to Reading. In December 2008, he was again signed on an emergency loan, this time for
Brentford, whose first-choice keeper
Ben Hamer, also borrowed from Reading, was suspended. He was named MOTM and Team of the week for his performance in this one game. On 20 September 2014, he made his full debut for the club in a 1–0 defeat away to
Sheffield Wednesday.
Bristol Rovers (loan) On 1 September 2009, deadline day, Andersen was signed on loan by Bristol Rovers until 1 January. He made his debut against Millwall which Bristol Rovers won 2–0, keeping a clean sheet. On 20 March, Andersen played against two other on-loan Reading players
Scott Davies and
Alex McCarthy. He kept a clean sheet from that game which ended 3–0 to Bristol Rovers. On 5 April, he saved a penalty from
Milton Keynes Dons'
Jermaine Easter as Rovers won 1–0, keeping another clean sheet this penalty save was his first ever one. Andersen played 58 League games for
The Pirates. He was awarded the League One side's Young Player of the Year trophy, and was also ranked third in the overall Player of the Year poll. For his performances and awards he was named in the BBC Team of The Year as well. Andersen finished his loan spell with Bristol Rovers on 5 January 2011.
Portsmouth (loan) On 17 August, Andersen joined
Portsmouth on loan for one month. He made his debut in an
Npower League 1 game against
Bournemouth, the game ended in a 1–1 draw.
Randers (loan) On 11 June 2013, Andersen joined
Danish Superliga side
Randers on loan for the
2013–14 season. Andersen made his debut on 20 July 2013 against
Viborg in a game that ended 2-2.
Midtjylland On 25 June 2015, Andersen signed a two-year contract with
FC Midtjylland. Andersen played all six matches in the
Europa League group stage against
Club Brugge,
Legia Warsaw and
Napoli, where FC Midtjylland finished as runners-up to qualify for the final rounds. In February 2016, Midtjylland and Andersen beat Manchester United 2–1 at
MCH Arena and lost 1–5 at
Old Trafford in the second leg of
round of 32. In those two matches, Andersen saved a penalty from
Juan Mata, and his general performances were generally lauded. In the same
2015–16 season, Andersen kept an average score of under one goal against in the Superliga, where Midtjylland finished third in the league and won bronze medals. Andersen left the club in the summer of 2017.
Lyngby BK Andersen signed a three-year contract with
Lyngby Boldklub on 24 June 2017.
SK Brann Andersen joined the Norwegian
Eliteserien club
SK Brann, in June 2021, but terminated his contract 20 August the same year, after a scandal involving him and other players partying at
Brann Stadion.
Viborg On 31 August 2021, Andersen joined newly promoted
Danish Superliga club
Viborg FF on a deal until June 2023. In June 2023, Andersen's departure from the club was confirmed.
Fremad Amager On August 4, 2023, Andersen was suddenly on the team sheet and in the starting lineup when the newly relegated
Danish 2nd Division club
Fremad Amager met
HIK in a league match. However, there had been no official announcement from the club about Andersen's move to Fremad Amager. In March 2025 the club confirmed, that Andersen had extended his contract for the upcoming three seasons. However, on 27 May 2025—just under two months later—the club announced that the contract with Andersen, who served as both player and goalkeeping coach, had been terminated with immediate effect, without further explanation. ==International career==