Mary Catherine McCormack (born February 8, 1969) is an American actress. 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), as Mary Shannon in In Plain Sight (2008–12), and as Casey in the AMC series Loaded.
Her film roles include Private Parts (1997), Deep Impact (1998), True Crime (1999), High Heels and Low Lifes (2001), K-PAX (2001), Dickie Roberts: Former Child Star (2003), 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. Her sister Bridget is a justice of the Michigan Supreme Court and a current professor of law at the University of Michigan Law School. Her brother William is an actor who has appeared in a recurring role as FBI agent Robert O'Connor on In Plain Sight.