McCormack's film roles include Private Parts (1997), Deep Impact (1998), True Crime (1999), Mystery, Alaska (1999), K-PAX (2001), Dickie Roberts: Former Child Star (2003) and 1408 (2007).

[1] She is the daughter of clinical therapist Norah and car dealership and ice cream parlor owner William McCormack.

[5] Her brother William is an actor, writer and producer[6] who has appeared in a recurring role as FBI agent Robert O'Connor on In Plain Sight.

[7] McCormack began her acting career at age 12, performing in Gian Carlo Menotti's Christmas opera Amahl and the Night Visitors.

[3] In 2004, McCormack joined the cast of The West Wing as Deputy National Security Advisor and ex-CIA officer Kate Harper.

[19] In October 2013, McCormack started filming scenes for The Crash (previously titled A Conspiracy on Jekyll Island) alongside Frank Grillo, AnnaSophia Robb, Minnie Driver, Dianna Agron, John Leguizamo, Ed Westwick, Christopher McDonald and Maggie Q.

[25] In 2018, McCormack began starring in the ABC sitcom The Kids Are Alright as Catholic family matriarch Peggy Cleary.

McCormack is a frequent celebrity panelist on the show, as are fellow executive producers Lisa Kudrow and Dan Bucatinsky.