Swank was born in Lincoln, Nebraska, the daughter of Judy (née Clough), a secretary and dancer, and Stephen Swank, who was an officer in the Air National Guard and later a traveling salesman. She has a brother, Dan. Many of her family members hail from Ringgold County, Iowa. Swank came from humble beginnings, particularly as a child growing up in a trailer park near Lake Samish in Bellingham, Washington, to which she moved at age six, after having lived in Spokane, Washington. Swank has described her younger self as an "outsider" who felt that she belonged "only when [reading] a book or [seeing] a movie, and could get involved with a character," and was thus inspired to become an actress.

When Swank was nine years old, she made her first appearance on stage starring in The Jungle Book. She became involved in school and community theater programs, including those of the Bellingham Theatre Guild. She went to Sehome High School in Bellingham until she was sixteen. Swank also com