Morrison was born in Chicago, Illinois and raised in Arlington Heights, Illinois. Her father, David L. Morrison, is a music teacher who was named Teacher of the Year by the Illinois State Board of Education in 2003.[1] Morrison's mother, Judy Morrison, was also a teacher. Both have since retired. She graduated from Prospect High School in 1997 where her parents also worked. During her time there, she played clarinet in the school’s marching band, sang in the choir and did cheerleading with the school’s pep squad. Morrison also has two younger siblings, Julia Morrison and Daniel Morrison. After high school, she attended Loyola University Chicago where she majored in Theatre and minored in English before graduating in 2000. She studied with the Steppenwolf Theatre Company before moving to Los Angeles to pursue a career in film and television.

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 Stonein the 1994 film, Intersection, and later appeared as Samantha in Stir of Echoes with Kevin Baconin 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 Affleckand James Gandolfiniand 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 Laurie In 2006, she starred in and produced the independent film Flourish.

Morrison has also appeared in music videos for Nick Lachey("Shut Up"), The Donnas ("Too Bad About Your Girl") and Jack's Mannequin ("Dark Blue"). She has also appeared on two TV Guide covers with her House co-stars, and has been featured in photoshoots with In Style, Stuff, Entertainment Weekly and Premiere among others. She is currently represented by Abrams Artists Agency.

In January 2007, People magazine reported that Morrison had become engaged to Jesse Spencerin Paris over the holidays.In a post-Golden Globes interview with E!, Morrison revealed that Spencer proposed on December 23, 2006, at the Eiffel Tower. According to an interview with Spencer and Morrison in In Style Weddings in July 2007, the couple met for the first time at Vancouver International Airport in March 2004 when they were on their way to shoot the first episode of House before they started dating in July 2004. In August 2007, however, the couple announced that they were calling off their engagement