In the early 1960s, Marsden worked with the Royal Shakespeare Company and began to accumulate an extensive list of theatrical credits that include everything from Anton Chekhov and Henrik Ibsen to contemporary Soviet playwright Alexander Vampilov.
His appearances include Crispen in The Friends, 1970; Casca and Lucilius in Julius Caesar, 1972; Paul Schippel in Schippel, 1974; Heinrich Krey in The Plumber's Progress, 1975.