In an unusual turnabout from the pattern typical for ingenues, Marsh underwent a series of rhinoplasties following her early successes rather than changing her appearance before starting her career. Already a pretty woman, the ultimate result was exceptionally dramatic and opened the door to more glamorous parts in the later 1960s. She was a frequent guest star on television into the 1970s; her last credited roles were in 1979.

In 1964 she played Ophelia in John Gielgud's celebrated Broadway production of Hamlet starring Richard Burton. Her Ophelia received mixed notices, but Gielgud liked her performance and resisted efforts to recast the part despite holding more auditions during rehearsals.

Marsh became one of the attractive young actresses who were regularly romanced by the stars of popular TV series, including The Man From U.N.C.L.E. (S3E21, "The It's All Greek to Me Affair", 1967 Feb 03), I Spy, The Wild Wild West (S1E14, "The Night of the Howling Light", 1965 Dec 17), It Takes a Thief (S1E11, "To Steal a Battleship", 1968 March 26), Hawaii Five-O (S02E07, "Sweet Terror", 1969 Nov 05; S03E07, "Forces of Waves," 1970 Oct 28; S12E06, "Image of Fear", 1979 Nov 08), and Daniel Boone (S6E19, "A Matter of Vengeance", 1970). Among her early television appearances she played Elizabeth Bacio, daughter of the title character, in the 1965 Perry Mason episode, "The Case of the Sad Sicilian." Appeared as Nora in The Big Valley (S4E16, "The 25 Graves of Midas", 1969 Feb 3).

Marsh's few film appearances included Che! (1969), Homebodies (1974), Freebie and the Bean (1974) and The Dark Secret of Harvest Home (1978).