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 (2002) (TV), 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" (1996) and "Sex, Love & Secrets" (2005), 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 (2006/I). After the release of a teen horror film, When a Stranger Calls (2006), in which she co-starred as Tiffany, Cassidy went to Vancouver to work on her second horror film, Black Christmas (2006), 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 (2009), the big-screen remake of the cult TV series.

Kate co wrote her songs with Garth Brooks in house writers, but Kate felt there were too much country influence in the music and not enough rock. Kate did not think they were listening to her idea’s enough, so she decided to return back home to her native homeland, Scotland. At the beginning of 1997 Kate bought her first guitar and taught herself how to play Kate then produced her first album "In Your Hands" with the help of local musicians from her home town of Glasgow. From Kate Cassidy's growing reputation and brilliant live performances through out the UK the public gave Kate the title "Scotland's Queen of Rock", and is much respected as a unique singer/songwriter from her appearances at "T" in the Park, King Tuts, The Boarder Line, Edinburgh Music Festival Dynamic Earth, World Art Stage, The Canons' Gait, BBC Music Live Event, Aberdeen's Alternative Festival, The Lemon Tree, Nozstock, plus TV and Radio. In 2002 Kate received an email from Mike Burn with an invitation to come to Germany. Six months later Kate landed in Frankfurt to meet MBAF for the first time with her acoustic guitar. MBAF had organised a few acoustic gigs for Kate, where the audiences were amazed with her voice and so were MBAF. It was going to be a further 15 months before Kate was to return to Germany due to other commitments. In the spring of 2003 Kate returned to Germany where Kate and MBAF have rehearsed and have played numerous gigs in Frankfurt and the surrounding area to the delight of the audiences that have been surprised and fascinated, at the ability of this voice and the force from were it comes. It is electric, powerful and alluring, Kate's Style of Rock R&B song writing and self-determination, has given her an edge that is more like a one woman rock band, There are many fantastic voices you could compare Kate Cassidy to; Sheryl Crow, Joan Baez, Bonny Raite, Aretha Franklin, Maggie Bell, Tina Turner, Tori Amos, Melissa Etheridge and Alanis Morissette, but none more unique than this Scottish artist!