Katie Cassidywas born in Los Angeles, California, the daughter of '70s pop idol David Cassidyand former model Sherry Benedon; the two had an "on-again, off-again" relationship during the 1970s-1980s, resulting in Katie's birth, fifteen years after first meeting. Cassidys paternal grandparents were actors Jack Cassidyand Evelyn Ward. She grew up in Calabasas, California with her mother and stepfather, Richard Benedon, an ER physician. She now has a close relationship with David Cassidy As a child, Cassidytook to tumbling and eventually became a cheerleader for the California Flyers, a competitive team. She cheered as a freshman for Calabasas High School where she attended classes and graduated in 2005. As a child, Cassidybecame interested in show business. She is also known for dating two members of former boy band Dream Street, rocker Greg Raposo (2002 - 2003) and popstar Jesse McCartney (2004-2007). Raposo's 2003 track "We're In Love" was written for her 16th birthday; it is also rumored that McCartney's album Right Where You Want Me is about her.

Cassidystarted community theater when she was in elementary school. She modeled for Abercrombie and Fitch and Rock and Republic's line in 2004, which helped open the door to an acting career. Her mother, Sherry, would not allow Katie to act professionally until completing her high school education. After high school, Katie was planning to attend NYU's Tisch School of Performing Arts, but instead moved to Los Angeles & landed a number of jobs on popular Hollywood TV shows, including a recurring role as Zoe on 7th Heaven. She then guest-starred in the one-hour drama Sex, Love & Secrets, but the series was canceled after a few episodes. In 2002 she released a single called "I Think I Love You". Her 2006 projects included a small role in the horror remake When a Stranger Calls and Click with Adam Sandler. She has also starred in another horror remake, the murderous Christmas classic Black Christmas. She has several films awaiting release.

She completed filming in Paris, where she worked on a film called Taken with Liam Neesonand Maggie Grace She is scheduled to star in a movie remake of the hit television series Dallas as Lucy Ewing, beating out Lindsay Lohanand Jessica Simpson Filming for Dallas is scheduled to begin sometime in 2008. Cassidyhad a 1 year-deal on popular The CW drama Supernatural.