Christensen was born in Vancouver, British Columbia, to Alie, an American speechwriter, and David Christensen, a Canadian computer programmer and communications executive. His father is of Danish descent, and his mother has Swedish and Italian ancestry. Christensen is one of four children, with three actor siblings: older brother Tove, older sister Hejsa, and younger sister Kaylen. Christensen was an athlete in high school, playing hockey competitively and tennis on a provincial level.

On May 12, 2000, Christensen announced that he would be starring as Anakin Skywalker in Star Wars: Episode II - Attack of the Clones (2002) and Star Wars: Episode III - Revenge of the Sith (2005). The casting director reviewed about 1,500 other candidates before director George Lucas selected Christensen. Lucas is quoted as saying that he chose Christensen because he "needed an actor who has that presence of the Dark Side". This was essential to solidify the story that Lucas was trying to tell: Anakin Skywalker's fall from grace and transformation into Darth Vader. During the production of Revenge of the Sith, Christensen asked Lucas if a special Vader suit could be constructed to fit his own body, rather than have a different actor don one of the original sets of Vader armor worn by David Prowse. Lucas agreed, and a suit was engineered to fit Christensen's frame, even including extensions to allow for the actor to attain Vader's 6 ft 6 in (1.98 m) height. His voice as the "robotic" Vader, however, was dubbed over by James Earl Jones, who first made the voice famous in the original trilogy. Stock footage of Christensen was used in the 2004 DVD-release edition of Return of the Jedi, where he was inserted to replace Sebastian Shaw as the ghost of the redeemed Anakin Skywalker. This was one of the most controversial changes. Lucas wanted Anakin's inner person to return to who he was before he turned to the dark side. Christensen insisted this was done without his knowledge, an act that was confirmed by Lucasfilm itself in the featurette "Return of the Jedi: What has changed?" as seen on the official website to commemorate the 2006 DVDs. His performance in both Episode II and III received generally mixed reviews by critics (earning him the Golden Raspberry Award for Worst Supporting Actor in both films). Christensen was named in both People magazine's 50 Most Beautiful People and Teen People's 25 Hottest Stars under 25. For his performance in Episode III, he won the MTV Movie Award for Best Villain.

Did they make that special suit?
Lucas agreed,