Katie Cassidy was born Katherine Evelyn Cassidy on November 25, 1986, in Los Angeles, California. Her father, David Cassidy, was a pop star in the 1970s. Her mother, Sherry Williams, was a high fashion model in the early 1970's. Sherry left her career to concentrate on raising her daughters, with husband, Richard Benedon M.D., Katie's step-dad. Her famous relatives include uncles Shaun Cassidy and Ryan Cassidy, and grandparents Jack Cassidy, Evelyn Ward and Shirley Jones.

As a child Katie took gymnastics, studied piano, guitar and singing, then took dancing and acting lessons and eventually became a cheerleader for the California Flyers. She attended Calabasas High School in her neighborhood, where she also cheered as a freshman. Her mother insisted that she complete college prep classes in high school before she embarked on her acting career. For years prior to graduation in 2005, Katie was modeling and studying acting in community theater.

Katie's early exposure to show business, the cosmopolitan culture of Los Angeles, her family upbringing, and her comprehensive education altogether produced a remarkable result. She grew up as a multifaceted person showing her many talents in a variety of professions such as recording her father's 1970s hit "I Think I Love You" and singing in VH1's TV special Bubblegum Babylon, writing her own songs, modeling for Abercrombie and Fitch and Guess?'s Rock and Republic clothing lines in 2004, and acting. However, she was publicly criticized by her father who would have liked for her to wait before she covered his classic hit. During her stint with Artemis Records, she was introduced to the popular boy band Dream Street. There she met Greg Raposo, who wrote the song "We're In Love" for her when they dated in 2002, She also met her boyfriend, Jesse McCartney, there and later appeared as his love interest in his video hit "She's Not You". A story on Jesse and Katie appeared in the January 2006 issue of Teen People magazine. At that time she was busy working on five feature film productions going on almost simultaneously.

Katie made several appearances in TV series, such as 7th Heaven and Sex, Love & Secrets, among others. She had to wear gigantic boobs in the small role of Samantha, the grown-up daughter of Adam Sandler and Kate Beckinsale in the comedy Click. After the release of a teen horror film, When a Stranger Calls, in which she co-starred as Tiffany, Cassidy went to Vancouver to work on her second horror film, Black Christmas, in which she stars as a sorority girl. Katie recently beat out the likes of Jessica Simpson and Kristin Cavallari for the leading role opposite John Travolta as his daughter and the heir to the most powerful energy company in the world in Dallas, the big-screen remake of the cult TV series.
Started off her career modeling with Abercrombie and Fitch. Daughter of David Cassidy. Did a cover of her father's song, "I Think I Love You". Granddaughter of the late Jack Cassidy. Has appeared in two remakes of two classical movie thrillers: When a Stranger Calls (originally released in 1979) and Black Christmas (originally released in 1974). Has 2 older sisters, Jenna & Jaime.