Bilson was born in Los Angeles, the daughter of Janice (née Stango), a sex therapist, and Danny Bilson, a writer/director/producer.Her father is Jewish and her mother, a native of Philadelphia, is Italian American.Bilson's father comes from a show business family.
Her great-grandfather, George Bilson, (born in Leeds, West Yorkshire, England) was the head of the trailer department at RKO Pictures, her Brooklyn-born great-grandmother, Hattie Bilson, was a screenwriter, and her grandfather, Bruce Bilson, is a film director Bilson's parents divorced when she was nine years old, and in 1997, her father remarried Heather Medway, an actress and the mother of Bilson's half-sisters Hattie and Rosemary.
Bilson had what has been described as a "self-destructive, rebellious period" during her teen years.When she was 14, she and a group of her brother's friends were involved in a car accident, a head-on collision with another carAs a result, Bilson was unconscious for a few days, has a scar above her right eye, and sometimes suffers from migraines and memory loss. She has stated that the experience "changed" her encouraging her to "stop... getting into trouble and stopping her from "going down that road" and because of this experience she learned to play the piano Bilson graduated from Walter Reed Middle School in 1995 and from Notre Dame High School in 1999. During her time at Notre Dame, she appeared in productions of Bye Bye Birdie, Once Upon a Mattress and The Crucible.
Bilson attended Grossmont College in a suburb of San Diego, but dropped out after one year taking her father's advice to pursue a professional acting career She then made several appearances in commercials, including advertisements for Subway restaurants, Raisin Bran and Pepto-BismolShe made her screen acting debut in early 2003, appearing in one episode of Buffy the Vampire Slayer and 8 Simple Rules for Dating My Teenage Daughter. Bilson was subsequently cast in The O.C.
Which debuted in August 2003. Her character, Summer Roberts, was initially intended to appear in only a few episodes, but became a series regular after a successful run, as Bilson's on-screen romance with Seth Cohen (Adam Brody) became a noted aspect of the series