She first appeared on stage in 2009 in Up at Steppenwolf Theatre followed by her Broadway debut in The Big Knife with Roundabout Theatre Company in 2013. In 2016 she played Desdemona in Othello at New York Theatre Workshop opposite David Oyelowo and Daniel Craig.

Brosnahan attended Wayne Thomas Elementary School, and then Northwood Junior High School. She performed in musical theater during junior high school and high school. At Highland Park High School, she was on the wrestling team for two years and was a snowboarding instructor. When Brosnahan was 16, she took a class with Carole Dibo, director of Wilmette’s Actors Training Center and now Brosnahan’s manager. Brosnahan graduated from New York University’s Tisch School of the Arts in 2012.

In 2013, she starred in the critically acclaimed Netflix political drama House of Cards with Kevin Spacey and Robin Wright. Although she was initially booked for only two episodes, she caught the eye of showrunner Beau Willimon, and her character was significantly expanded. Her role brought her career prominence and received a Primetime Emmy Award nomination for Outstanding Guest Actress in a Drama Series.

In 2016, she appeared in Woody Allen’s Amazon series Crisis in Six Scenes with Allen, Elaine May, and Miley Cyrus.

Since 2017, she has appeared as the title character in the Amazon period comedy series, The Marvelous Mrs. Maisel. Brosnahan said that, while she is not Jewish, her exposure to Jewish culture growing up helped prepare her for the role of Midge Maisel, a 1950s Jewish housewife who pursues a career in stand-up comedy when her marriage falls apart. Her performance earned her a Primetime Emmy Award, two Golden Globe Awards, two Screen Actors Guild Awards, and two Critics’ Choice Television Awards, among others.

It was reported in 2018 that Brosnahan had married actor Jason Ralph, but she later revealed in January 2019 that they had been married “for years” before their relationship became public. Both attended the 76th Golden Globe Awards ceremony in 2019, where she thanked him during her acceptance speech.

Brosnahan appeared in the drama film The Courier, alongside Benedict Cumberbatch. The film was released in January 2020.