Bilson's character, Summer Roberts, was intially 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.
Bilson was born to an Ashkenazi Jewish American father (writer/director/producer Danny Bilson) and an Italian American mother. Her father comes from a showbusiness family, as her great-grandfather, George Bilson, was the head of the trailer department at RKO Pictures, while her great-grandmother, Hattie, was a screenwriter and her grandfather, Bruce Bilson, is a film director. Bilson's parents divorced during her childhood, and in 1997, her father remarried to Heather Medway, an actress and the mother of Bilson's half sister Hattie (born December 19, 2001).
Bilson grew up with her father's family. When she was fourteen, she and a group of friends experienced a car accident, involving a head-on collision with another car. As a result, Bilson has a scar above her right eye, and sometimes suffers from Migraines and memory loss; she has stated that the experience encouraged her to "stop... getting into trouble". Bilson graduated from Walter Reed Middle School in 1996 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.
At the age of seventeen, Bilson took her father's advice to pursue a professional acting career, and made several appearances in commercials, including advertisements for Raisin bran and Pepto-Bismol. She made her screen acting debut in early 2003, appearing in episodes of Buffy the Vampire Slayer, 8 Simple Rules for Dating My Teenage Daughter, and in the short film, Unbroken.
As a result of the success of The O.C., Bilson has become well-known among teenage audiences. In 2005, Maxim magazine named her in the sixth spot of their annual "100 Hot List". At the 2005 Teen Choice Awards, Bilson collected three awards - "Choice Hottie Female", "Choice TV Actress (Drama)" and "Best Onscreen TV Chemistry" (jointly won with Adam Brody). Her film debut will be in The Last Kiss, a romantic comedy starring Zach Braff and Jacinda Barrett. The film is currently scheduled for a release in October of 2006.
Bilson recently purchased Rose McGowan's home in the Hollywood Hills for $1.2 million. She has been dating her The O.C. co-star, Adam Brody, for several years; In 2005, Brody gave Bilson a pit bull dog, Penny Lane, as a birthday gift.
Bilson is recognised by some to have a noted fashion sense, and has sported outfits by designers Thakoon and Marc Jacobs. She enjoys watching television game shows, especially Jeopardy!