Foster's professional stage debut came in 1952 as Lorenzo in "The Merchant of Venice" in County Cork. In 1955, he made his London stage debut as the Electrician in "The Night of the Ball" at the New Theatre (now the Noël Coward Theatre). His first film role was in "The Battle of the River Plate" (1956), as part of the crew of HMS "Exeter", in which he played Able Seaman Roper. Over the next decade and a half, he performed in Joseph Losey's "King and Country" (1964), "The Family Way" (1966), "Robbery" (1967), "Inspector Clouseau" (1968), "Battle of Britain" (1969), and David Lean's "Ryan's Daughter" (1970). He had a regular role on the TV series "The Troubleshooters" (1965). In 1970, he played a Fenian revolutionary paramilitary leader in David Lean's epic film "Ryan's Daughter".