During the summer of 2008, Fahy attended open calls and was cast as the standby for Jennifer Damiano as Natalie Goodman in Arena Stage's production of Next to Normal in late 2008.

[2] On July 19, 2010, after Damiano departed the cast to prepare for the Broadway musical Spider-Man: Turn Off the Dark the day prior, Fahy took over as the principal actor for Natalie along with Marin Mazzie as mother Diana and Jason Danieley as father Dan.

She has also recorded demos for composers' works, such as Bree Lowdermilk and Kait Kerrigan's The Unauthorized Autobiography of Samantha Brown in 2009, and continues to perform in concert revues as the title character as recently as March 1, 2011.

[7] In May 2011, Fahy briefly reprised her role as Natalie Goodman in the first national tour of Next to Normal in the first week of the St. Paul, Minnesota stop.

Fahy joined the rest of the principal tour cast for her stint: Alice Ripley as Diana, Asa Somers as Dan, Jeremy Kushnier as Dr. Madden, and Preston Sadleir as Henry.

[16] The following year, Fahy appeared in the miniseries Political Animals as ambitious blogger Georgia,[17] and guest starred on Necessary Roughness as Olivia DiFlorio, tutor-turned-girlfriend of the main character's son.

[22] In February 2022, Fahy was cast in a starring role playing Daphne Sullivan, a woman vacationing with her husband, in the second season of the HBO drama anthology series The White Lotus.

However, her first stage role was playing Dorothy Gale in her Longmeadow High School senior class performance of The Wizard of Oz.