The brother of actor/director Basil Dignam, Mark Dignam studied journalism before making his 1930 stage bow in Lonely House. After a few seasons with the Ben Greet repertory troupe, he made his London debut in 1932 and thereafter worked with such organizations as the Royal Shakespeare Company and the National Theatre. In films from 1949, Dignam specialized in roles calling for gray-maned authority, notably the Laird in The Maggie (1954), the prosecuting attorney in Carrington, V.C. (1955), and Merlin in Cornel Wilde's Sword of Lancelot (1962). Mark Dignam reluctantly earned a bit of notoriety in 1969 when he appeared as Polonius in Nicol Williamson's controversial film version of Hamlet.