Tunney was born in Chicago, Illinois, to a car salesman father, Patrick, and a bartender mother, Cathy. Tunney's father was born in Straide, County Mayo, Ireland, and her maternal grandparents were from Clare Island, Ireland. She is a cousin of Chicago Alderman Tom Tunney.
Tunney grew up in Orland Park, a southwest suburb of Chicago. She was raised Irish Catholic, attended Carl Sandburg High School, Saint Ignatius College Prep in Chicago and the Chicago Academy for the Arts in Chicago, and resided in Palos Heights, also in the Chicago area.