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. 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.