Andersen made his senior debut for Hvidovre IF in the lower league Danish 1st Division, and was recognized as a big talent.

[2] He made some errors in his first time at the club, dropping against Odense Boldklub[5] and Viborg FF,[6] but was kept in the starting line-up and eventually upped his game.

On 19 May 2014, it was announced he would return to Denmark on a permanent deal, signing a three-year contract with Superliga side FC Copenhagen, the main rivals of his previous club Brøndby.

[12] Andersen was called up for the Denmark under-21 national team in June 2001, where he immediately displaced the two-years older Rune Pedersen.

Following strong displays for AB, Andersen made his Danish national team debut against Spain in a March 2004 friendly match.

While playing for Brøndby, Andersen got his second cap in September 2008, and replaced the injured Thomas Sørensen for three games in the 2010 FIFA World Cup qualification.

After the injured Thomas Sørensen was ruled out of UEFA Euro 2012, Andersen replaced him as the first-choice goalkeeper for the Danish national team.