Brosnahan was born in Milwaukee, Wisconsin.[4] Her father worked in children's publishing. Her mother is British.[5] From the age of four, she was raised in Highland Park, Illinois.[4][6] Brosnahan was cast in her first movie role as Lisa in the Michael Bay-produced horror movie The Unborn while she was still in high school. She graduated from Highland Park High School in 2008 and the New York University[7] Tisch School of the Arts in 2012.[8] She is the niece of handbag designer Kate Spade.[9]