Terrence E. Hardiman was born in Forest Gate, Essex, an area that would later become part of London, to a police officer. He attended school in Essex and graduated in 1956 before going on to study English at Cambridge University's Fitzwilliam College.
As an amateur thespian, Hardiman began his career on the Shakespearean stage with the Cambridge University Players and The Marlowe Society in the late 1950s. He gained recognition for his impressive early performance as Mephistopheles in Doctor Faustus, which led him to join the Royal Shakespeare Company in Stratford-Upon-Avon.
Hardiman was a member of the ensemble from 1966 to 1970, during which time he appeared in classic plays such as King Lear, Hamlet, and A Midsummer Night's Dream, where he played the role of Starveling the tailor in Peter Brook's minimalist production.
In addition to his work on stage, Hardiman began acting on screen in 1965, excelling in the portrayal of autocratic, imperious, or sinister characters. His most notable role was as the piercingly green-eyed, mind control-using antagonist of The Demon Headmaster, a children's sci-fi series that made him a recognized figure in and around London.
In real life, Hardiman was known for his amiable temperament and was referred to as a "good-natured gentleman". Prior to his role as the demonic principal, he had enjoyed recurring roles in Softly Softly: Task Force, Secret Army, and Granada's Crown Court.
Hardiman's impressive gallery of personae included abbots and cardinals, Nazi officers, doctors, judges, police superintendents, university professors, a grand wizard, and even a Prime Minister. He also provided narration and voice-overs for numerous audio books by authors such as Roald Dahl, Ken Follett, Colin Dexter, and Wilkie Collins.
Hardiman was married to actress Rowena Cooper for almost six decades before his death on April 18, 2023, at the age of 86.