Marsh's film credits include the Tony Hancock film "The Rebel" (1961), "Cleopatra" (1963) as Octavia, "Unearthly Stranger" (1964), "Charlie Bubbles" (1967), "The Limbo Line" (1968), Alfred Hitchcock's "Frenzy" (1972), "Dark Places" (1973), "The Eagle Has Landed" (1976), "The Changeling" (1980) and the fantasy films "Return to Oz" (1985) and "Willow" (1988). In 1994, she starred in a villain role in the Nickelodeon/Thames Television remake of "The Tomorrow People". Her television films include "Goliath Awaits" (1981), "See China and Die" (1981), "Master of the Game" (1984), "The Corsican Brothers" (1985), "A Connecticut Yankee in King Arthur's Court" (1989), "Fatherland" (1994) for which she won a CableACE award for supporting actress, and "The Pale Horse" (1997). From 1982 to 1983, she portrayed the part of Roz Keith in the American sitcom "9 to 5".