Her film roles include Private Parts (1997); Deep Impact (1998); True Crime (1999); High Heels and Low Lifes (2001); K-PAX (2001); Right at Your Door (2006) and 1408 (2007).

McCormack was born in Plainfield, New Jersey. She is the daughter of clinical therapist Norah and car dealership and ice cream parlor owner William McCormack. Her parents divorced in 1990. She has a sister Bridget, a professor of law at the University of Michigan Law School, and a brother William, an actor.

McCormack graduated from The Wardlaw-Hartridge School in Edison, New Jersey, in 1987 and went on to attend, but never graduate from, Trinity College in Hartford, Connecticut, majoring in English and painting.

McCormack married Brothers & Sisters producer Michael Morris in July 2003. They have two daughters Margaret (born 2004) and Rose (born 2007). McCormack is the godmother of Frederick Weller's daughter.

McCormack began her acting career at age 12 performing in Gian Carlo Menotti's Christmas opera, Amahl and the Night Visitors. She continued her stage work in many more musicals and finished an acting program at the William Esper Studio. She has worked in many New York theater productions, including The Atlantic Theater Company, Alice's 4th Floor and Naked Angels, where she appeared in Jon Robin Baitz's A Fair Country.

McCormack portrayed Justine Appleton on Murder One (1995–1997) and also starred in the HBO series K Street, which aired for only ten episodes. She starred in Howard Stern's movie Private Parts. In 2004, McCormack joined the cast of The West Wing as Deputy National Security Advisor and ex-CIA officer Kate Harper. Her character soon became a recurring role and remained throughout the last three seasons of the series until its finale.

McCormack now stars in a critically acclaimed television series broadcast on the USA Network, In Plain Sight. She plays Mary Shannon, a Deputy United States Marshal attached to the Albuquerque, New Mexico, office of the Federal Witness Security Program (WITSEC), more commonly known as the Federal Witness Protection Program. On July 28 2010, the show was renewed for its fourth and fifth seasons, to run in 2011 and 2012.

McCormack was nominated for the 2008 Tony Award for Best Performance by a Featured Actress in a Play for the role of Gretchen in the 2008 Broadway revival of Boeing Boeing.

SAG Award nomination for Outstanding Performance by an Ensemble in a Drama Series (2005)SAG Award nomination for Outstanding Performance by an Ensemble in a Drama Series (2006)

Prism Award nomination for Performance in a Drama Episode (2009)