In May 2008 he was linked with a £1 million move to West Bromwich Albion. For the 2009–10 season, FCK bought Johan Wiland as a backup. When Christiansen was injured against Brøndby IF in the seventh Superliga game of the season, Wiland replaced him with such success that Christiansen was permanently demoted to the bench. Media reports suggested he wanted a transfer, and when FCK bought Kim Christensen as a new backup keeper in the Summer 2010, Christiansen was expected to leave the club. He signed for IF Elfsborg of the Swedish Allsvenskan championship in June 2010. During his time with Vejle, Christiansen was called up for the senior national team by national team manager Morten Olsen. He was chosen for the Danish squad at the 2002 FIFA World Cup in South Korea and Japan as a backup for starting national team goalkeeper Thomas Sørensen, and was an unused substitute throughout the tournament. In his time at Viborg, Christiansen was recalled to the Danish national team, and made his national team debut on 2 June 2005, in a 1–0 friendly match win against Finland. He played his second game in October 2005, when he came on as a substitute in a 2006 FIFA World Cup qualification 2–1 win against Kazakhstan. He quickly established himself as an understudy for Thomas Sørensen, and made a number of appearances at times when Sørensen was injured. Before the 2010 FIFA World Cup, Christiansen lacked playing time at FCK, and found himself in contention with Kim Christensen for the spot as third goalkeeper in the squad for the finals. Eventually Morten Olsen chose Christiansen for his team, and he spent his time at the tournament as an unused substitute. After Christiansen left Vendsyssel FF in the summer 2017, he was officially announced as the new assistant manager of Danish 1st Division club AB. In January 2018, he left AB to become goalkeeper coach at Fremad Amager. In 2019, he also functioned as assistant manager for a short period. On 15 July 2019 it was announced, that Christiansen had left Fremad Amager because he wanted to stay closer to his family and then became the new goalkeeper coach of Viborg FF. In November 2019, Christiansen was on the bench for Viborg in three games, after first keeper, Ellery Balcombe, was banned with a red card and the third and fourth choices was out with injuries, which left Viborg with only Ingvar Jónsson who was able to play.