Feldstein made her acting debut by appearing in the ABC comedy series My Wife and Kids (2002). In 2012, she played Megan in the musical television pilot Madison High.
In 2015, Feldstein appeared as a guest in the third season of the Netflix's comedy series Orange Is the New Black. She played the role of Anna in the film Fan Girl.
On October 18, 2016, it was announced that Feldstein would play Minnie Fay in the 2017 Broadway production of Hello, Dolly!, alongside Bette Midler, making her Broadway debut. The show began previews on March 15, 2017 and opened on April 20, 2017.
Feldstein also co-starred in Greta Gerwig's solo directorial debut Lady Bird, which was released to critical acclaim in 2017.
