Raised in Thornhill, Ontario,outside Toronto, Christensen attended Baythorn Public School for his public schooling as well as Unionville High School in Unionville, Ontario, Canada that is well-respected for its outstanding arts program. He was "discovered" when his older sister Hejsa, a former trampoline champion, was shopping around for an agent after she landed a role in a Pringles potato chips commercial.
Christensen's first acting role was on the television series Family Passions, which aired on Canadian television in September of 1993. The following year he acquired a minor role in John Carpenter's In the Mouth of Madness. In 1995 he had minor roles in Street Law and television films Love and Betrayal: The Mia Farrow Story and Harrison Bergeron.
Christensen was the subject of part of London photographer Sam Taylor-Wood's photographic series called Crying Men (2002-2004), which also features 26 other actors in private moments of reflection. He was named one of People Magazine's "50 Hottest Bachelors" of 2005 and was chosen by Teen People as one of the "25 hottest stars under 25" in 2005.