At Georgetown, Marling met her long-time collaborators, Future Directors Mike Cahill and Zal Batmanglij. Marling spent her junior year summer interning for the investment banking firm Goldman Sachs as an investment analyst. She felt a lack of meaning a life spent there would have and later turned down a job offer from the firm, opting instead to move to Cuba with Mike Cahill to film the documentary Boxers and Ballerinas. The documentary gained Marling recognition in 2004, having co-written the film with Mike Cahill and Nicholas Shumaker and co-directed with Cahill.

In 2005 Marling moved with Cahill and Batmanglij to Los Angeles. Marling went to auditions and was offered roles in horror films, but rejected them. She has stated she "wanted to be able to cast herself in roles that wouldn’t require her to play the typical parts offered to young actresses, the perfunctory girlfriend or a crime victim." She was discovered by talent agent Hylda Queally.

In the summer of 2009 she joined a group of freegans with friend Zal Batmanglij, living in tents and retrieving food from dumpsters, to explore how other young people were constructing a meaningful life.

Marling co-wrote, co-produced, and acted in the 2011 films Sound of My Voice and Another Earth, directed by Batmanglij and Cahill respectively. Both of these films were featured at the 2011 Sundance Film Festival, with Another Earth winning the Alfred P. Sloan Prize for outstanding film with science, Technology or math as a major theme. In 2012, she played Richard Gere's daughter in Arbitrage.

In 2013 she collaborated with Searchlight once again as co-writer and leading role in The East alongside Ellen Page and Alexander Skarsgård. Directed by Zal Batmanglij and co-written by Marling and Batmanglij, The East is based on the duo's experience as freegans and their concern with the side effects of prescription drugs.

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.

Marling and Batmanglij collaborated to create drama series The OA that debuted in 2016 on Netflix. It was written by Marling and Batmanglij, who produced the series along with Dede Gardner and Jeremy Kleiner of Plan B, and Michael Sugar of Anonymous Content.

Despite her many roles in films she has co-written, Marling stated she "get[s] a lot more pleasure in acting in other people’s stories" since "one of the great pleasures of acting is surrendering to someone else's point of view of the world."