[12][13] While still in high school, Brosnahan received her first movie role—that of Lisa in the Michael Bay-produced horror film The Unborn (2009).

In college, she appeared in single episodes of television series such as Gossip Girl, The Good Wife, Grey's Anatomy,[11] and In Treatment.

Brosnahan's early roles include performances in two short films directed by Ari Aster, Basically (2013), and Munchausen (2014).

After college, she began landing recurring roles in series such as the crime thriller The Blacklist (2014) and the short-lived medical drama Black Box (2014).

[15] In 2013, she starred as Rachel Posner in the critically acclaimed Netflix political drama House of Cards with Kevin Spacey and Robin Wright.

Her role brought her career prominence, and she received a Primetime Emmy Award nomination for Outstanding Guest Actress in a Drama Series.

In 2023, she starred in the revival of Lorraine Hansberry's The Sign in Sidney Brustein's Window opposite Oscar Isaac first at the Brooklyn Academy of Music, and later produced at the James Earl Jones Theatre on Broadway directed by Anne Kauffman.

In 2025, Brosnahan will star as Lois Lane in the DC Universe superhero film Superman, from director James Gunn.