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 British comedy/drama series Loaded by Channel 4.

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).