Hardwicke acted at the Bristol Old Vic, The Oxford Playhouse, and the Nottingham Playhouse. In 1964, he joined the National Theatre. He performed there for seven years. He appeared in William Shakespeare's "Othello", Ibsen's "The Master Builder". Peter Shaffer's "The Royal Hunt of the Sun", "Charley's Aunt", Tom Stoppard's "Rosencrantz and Guildenstern Are Dead", Congreve's "The Way of the World", Georges Feydeau's "A Flea In Her Ear", "The Crucible", Luigi Pirandello's "The Rules Of The Game", Fyodor Dostoevsky's "The Idiot", and George Bernard Shaw's "Mrs. Warren's Profession". In 1977, Hardwick went back to the National Theatre to perform in Feydeau's "The Lady from Maxim's"