AJ Cook (2000) - said to be "just friends"
Natalie Portman (2001) - said to be "just friends" did date and spotted holding hands
Kate Fischer (2003) - an Australian civilian confirmed by paparazzi photos
Lola Skye (2005) - confirmed by paparazzi photos
Eva Longoria (2005) - rumored fling at Cancun film festival per NY Daily
Sienna Miller (2006) - confirmed by paparazzi photos
Rachel Bilson (2007 to present) - confirmed by paparazzi photos

Christensen was born in Vancouver, British Columbia, Canada, the son of Alie, a speechwriter for the heads of large companies, and David Christensen, a software program writer and communications executive. His father is of Danish descent and his mother has Italian and Swedish ancestry. Christensen has an elder brother and sister, Tove and Hesja, and a younger sister, Kaylen. Raised outside Toronto in Thornhill, Ontario, Christensen attended E.J sand Public School, Baythorn Public School and Unionville High School in Unionville, Ontario, Canada.

Christensen's first acting role was on the television series Family Passions, which aired on Canadian television in September of 1993, at the age of twelve. The following year he acquired a minor role in John Carpenter's In the Mouth of Madness. From 1995 through 1999, he appeared in several movies and television shows, including Harrison Bergeron, Forever Knight, Goosebumps, The Virgin Suicides, and Are You Afraid of the Dark?.

He acquired wider notice while starring in Fox's Family Network TV series, Higher Ground in 2000, portraying a teen who was sexually molested by his stepmother, who then turned to drugs in his despair.