Christensen was born in Seattle, Washington, the daughter of Kathy, a construction manager, and Steven Christensen, an insurance worker/human-resources executive. She has an older half-brother, Nick, two younger twin brothers, Dane (who appeared in the film The Upside of Anger) and Brando. She is not related to Golden Globe Award-nominated Canadian actor Hayden Christensen. She has Swedish, Danish, Norwegian, Welsh, and Irish ancestry. Christensen was raised in Los Angeles, California and started out on television in commercials for McDonald's.

Christensen is a member of the Church of Scientology. Her parents became Scientologists during their 20s and raised her to be a Scientologist as well as home-schooling her.