Marling was born in Chicago, Illinois, the daughter of property developer parents John and Heidi Marling. She was named "Brit" after her Norwegian maternal great-grandmother. She has a sister, Morgan Marling, who has been dating Writer and Director Francesca Gregorini since 2014. She grew up in Winnetka, Illinois, and attended the arts program at Dr. Phillips High School. Marling was interested in acting, but her parents encouraged her to focus on academics. She graduated from Georgetown University in 2005 with degrees in economics and studio art, and was her class valedictorian.