Mary Catherine McCormack is an American actress. She has had leading roles as Justine Appleton on the series Murder One (1995–1997), as Deputy National Security Adviser Kate Harper on The West Wing (2004–2006), as Deputy U.S. Marshal Mary Shannon on In Plain Sight (2008–2012), and as Peggy on the comedy series The Kids Are Alright (2018–2019).

Murder One is an American legal drama television series that aired on ABC from September 19, 1995, until May 29, 1997. The series was created by Steven Bochco, Charles H. Eglee, and Channing Gibson.