[6] Christiansen signed a four-year contract with Barcelona in July 1991, when Johan Cruyff was team manager, with a dream of playing alongside their Danish playmaker Michael Laudrup.

[7] He started competing as a senior with the reserves,[8] where he soon was joined by compatriot Ronnie Ekelund; during this period, he was frequently called up to train with the main squad, but received almost no playing time due to stiff competition.

[11] Still playing for Barcelona's B team, Christiansen was called up for the Spain senior squad by manager Javier Clemente, in January 1993, making his debut against Mexico on the 27th and impressing in a game which featured few regulars for the European nation.

[18] Following a good start, he failed to score any goals in his second year even though he appeared in 31 matches, and was sold to Segunda División's Villarreal in November 1997, helping them promote[19] but managing to find the net only once the following campaign, which ended in relegation.

He went on to play for Terrassa FC in the Spanish lower leagues,[21] finishing that season at Panionios in Greece before returning to Denmark in August 2000, signing with defending Danish champions Herfølge, and showing good form when he scored two goals in a win over eventual runners-up Brøndby.

[24] He scored nine times in his first year, but failed to reproduce his previous form mainly due to several injuries, including a knee operation and two shinbone ailments; in the summer of 2006, the club chose not to prolong his contract and he left, retiring shortly after at the age of 33.

[25][9] Christiansen started his managerial career in the United Arab Emirates, as part of Luis Milla's coaching staff at Al Jazira, arriving in February 2013 and leaving in October as the latter was fired.

In late April 2014 he was appointed head coach of AEK Larnaca in the Cypriot First Division, after having been approached for the job by former Barcelona B teammate Xavier Roca, who acted as director of football;[26] in his first two seasons, he led them to consecutive best-ever runner-up league finishes.

[51] Later in June and July at the Gold Cup, they won on penalties against hosts the United States in the semi-finals in San Diego, but lost the final to Mexico by a single goal.