Cassidy started 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.

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