Colleen Rennison is of Irish, Scottish and French descent. She was born to Pat, a mortgage underwriter, and Cecilia Rennison, a television researcher, and was raised in Vancouver, British Columbia. She began her acting career with a role in the Canadian indie film Max in 1994. This was closely followed by appearances in a handful of television movies and a guest-starring role on the television series The Outer Limits. Carpool, starring Tom Arnold, David Paymer and Rachael Leigh Cook, was her first major American feature. Shortly after she landed a role in the John Dahl-directed Unforgettable with Ray Liotta and Linda Fiorentino, followed by Stargate SG-1 episode 'Bane' in 1998 with Richard Dean Anderson and Christopher Judge, followed by The Story of Us (1999), directed by Rob Reiner and featuring Bruce Willis and Michelle Pfeiffer, in which she portrays their daughter Erin. Her next major role was playing Mona Hibbard as a young girl (Minnie Driver played the character as an adult) in the film Beautiful, directed by Sally Field, also starring Joey Lauren Adams and Kathleen Turner.

Rennison continued to work on more Canadian projects such as the short lived but critically acclaimed These Arms of Mine for CBC, which landed her a Gemini Award nomination, and the Emmy Award winning casting Director Coreen Mayrs' independent film A Feeling Called Glory (based on the short story "The Presbyterian Sidewalk" by Barbara Gowdy), for which Rennison won a Gemini in 2001, making her the youngest ever winner at 13. In 2005, Chris Haddock (of Canadian drama "Davinci's Inquest"), who was familiar with her work, hand selected her to play a variation of Kelly Ellard (convicted killer of Reena Virk) for a recurring role in "Davinci's City Hall". Most recently Rennison has appeared in the films American Venus, Boot Camp with Mila Kunis, and What Goes Up starring Hilary Duff, Steve Coogan and Molly Shannon. Rennison has also appeared in the television series Highlander: The Series, Poltergeist: The Legacy, Millennium, and The Sentinel. She appeared twice on Stargate SG-1 as Ally and as Cassandra.

In 2005, Rennison's mother submitted a recording of Rennison to CityTV, a local station holding a contest. Out of thousands of entries, she was selected along with two other girls to meet with Grammy Award-winning and multi-platinum-selling producer/composer/songwriter David Foster at his Malibu, California, home (featured on the reality television series Princes of Malibu with stepsons Brody Jenner and Brandon Jenner). This meeting led to a performance later that year at the David Foster and Friends Celebrity Gala to support The David Foster Foundation in Vancouver.

In 2008, Rennison relocated to New York City to begin training at Circle in the Square Theater School in their two-year acting program, which she left halfway through after they refused to allow her into their musical theatre track because of her low range and untrained voice. While still in New York, she signed with a management team at Shady Records, but after 6 months terminated the relationship due to conflicting musical views.

She also released a solo album in 2014, entitled See the Sky About to Rain.