Mary Catherine McCormack (born February 8, 1969) is an American actress. Best known for her work in television, she has had leading roles as Justine Appleton in the series Murder One (1995–97), as Deputy National Security Adviser Kate Harper in The West Wing (2004–06) and as Mary Shannon in In Plain Sight (2008–present).
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.