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. During college, she performed in single episodes of TV series such as Gossip Girl, The Good Wife, Grey's Anatomy and In Treatment. After college, she began landing recurring roles in series such as The Blacklist (2014), the short-lived Black Box (2014), and House of Cards (2013–15) which brought her career prominence, including an Emmy nomination for Outstanding Guest Actress in a Drama Series in 2015. Initially booked for two episodes and five lines, she caught Beau Williamson's eye; soon, she was developed into a major character. She played Abby Isaacs for two seasons on the TV drama series Manhattan.