Morrison started her career as a child model, appearing in print advertisements for JCPenney and Montgomery Ward and commercials for Rice Krispies and Mondos. At the age of ten, she was featured on the cover of Sports Illustrated for Kids with basketball star Michael Jordan.

She made her film debut at the age of fifteen as the daughter of Richard Gere and Sharon Stone in the 1994 film, Intersection, and later appeared as Samantha in Stir of Echoes with Kevin Bacon in 1999. Her first leading role came in the 2000 film Urban Legends: Final Cut, and she has since gone on to appear in films including Grind with Adam Brody, Surviving Christmas with Ben Affleck and James Gandolfini and Mr. and Mrs. Smith with Brad Pitt and Angelina Jolie in 2003, 2004 and 2005, respectively.

On television, she has worked on several hit shows including Touched by an Angel and Dawson's Creek before landing the role of immunologist Dr. Allison Cameron on House in 2004 co-starring alongside Hugh L