His long career in the theatre, film and television spanned more than 70 years with memorable screen roles such as Siegfried in All Creatures Great and Small (1978–1980 and 1988–1990), and Sir Winston Churchill, most notably in Winston Churchill: The Wilderness Years (1981), for which he won a BAFTA award.
Roberty Hardy was also a respected historian, interested in medieval warfare. He wrote and presented an acclaimed television documentary on the subject of the Battle of Agincourt and wrote two books on the subject of the longbow, Longbow: A Social and Military History and The Great Warbow; From Hastings to the Mary Rose with Matthew Strickland. He was one of the experts consulted by the archaeologist responsible for raising the Mary Rose. In 1996, he was elected a Fellow of the Society of Antiquaries of London.