Christiansen started his senior career at Lyngby BK. He made his debut in the 2008–09 season, making three league appearances. The following season Christiansen earned more playing time and helped Lyngby achieve promotion to the top flight. He excelled as one of Lyngby's key players in the Danish Superliga for the following two seasons, but could not save the team from relegation in their second season.
Christiansen's play in the Superliga earned him attention from the reigning champions FC Nordsjælland. On 25 July 2012, Christiansen was presented as a Nordsjælland player after signing a four-year contract. He made 67 league appearances for the club during his two and a half year stint, and appeared in five games in the 2012–13 UEFA Champions League group stage.
On 15 January 2015, Christiansen secured a transfer to Serie A club Chievo Verona. During his year in Italy he failed to earn much playing time, appearing in just five games.
On 26 January2016, Christiansen was presented as Allan Kuhn's first signing as the newly appointed head coach at Malmö FF. He made his Allsvenskan debut in the season opener against reigning champions IFK Norrköping, where he scored his first goal for the club in a 3–1 win. Christiansen scored five more goals during the 2016 season and played a key role in Malmö's championship winning team until he suffered a season-ending injury with seven games to play. He recovered just in time for the start of the 2017 season, and soon picked up where he left off before the injury. On 7 September 2017, it was announced that Christiansen had extended his contract with Malmö FF over the 2022 season. On 16 October 2017, Christiansen scored a goal and added two assists in Malmö FF's 3-1 victory over IFK Norrköping, which secured the league title with three games to play. After the season, Christiansen was voted Allsvenskan's midfielder of the year and the league's most valuable player.
After a six month spell at Gent he rejoined Malmö FF in July 2018. Christiansen became club captain ahead of the 2020 season after predecessor Markus Rosenberg retired.
International career
Christiansen has represented the Danish national team in four friendlies. After a successful spring in Malmö he got called up by former Malmö coach Åge Hareide in May 2016, but had to decline after suffering a thigh injury.
