Christensen started in 1991 as a music producer and DJ, and had his first success in 1992 with U96 with Das Boot. In several countries this song was the first techno song to obtain a number one position. Since 2002 Christensen also works under the name Alex C. and frequently collaborates with Yasmin K. whom he met as a judge on the second season of Popstars.
As a producer and/or composer Christensen worked for big artists as Right Said Fred, Tom Jones, *NSYNC, Rollergirl, Marianne Rosenberg, Sarah Brightman, and ATC.
Christensen represented Germany at the Final of the 2009 Eurovision Song Contest in Moscow together with Oscar Loya, performing the song "Miss Kiss Kiss Bang" finishing in 20th place.
Alex Christensen's song "Doktorspiele" which featured additional vocals from Yass, won the 2008 Eurodanceweb Contest, which were determined by points assigned by a professional jury of disc jockeys, journalists, music producers, webmasters, and radio speakers from all over the World.
One of Christensen's produced songs, ATC's "Around the World (La La La La La)", was later sampled in Beat Ink's "Around the World" in 2008 and used in Chris Webby's "La La La" in 2009.
In 2007, Christensen released the song "Du hast den schönsten Arsch der Welt" in co-operation with Y-ass. It sampled The Soundlovers' "Run Away". This song marked an international breakthrough for Christensen. Because the song got high ratings in several foreign dance charts, it was re-released in several other languages. In some countries the melody was adjusted to fit the taste of the local inhabitants.