Although he did not turn to the stage until middle age after two decades in the Royal Navy, Jack Gwillim was a notable actor on both sides of the Atlantic. During the 1950s he was a member of the Royal Shakespeare company in Stratford, England, for three years during Sir Anthony Quayle's stint as artistic director, and a member of the Old Vic Company for a further three years. His extensive theatre credits included many leading roles in the West End including "Sacred Flame" with Gladys Cooper and Wendy Hiller, "The Right Honourable Gentleman" with Quayle, "The Dark is Light Enough" with Dame Edith Evans, "Castle In Sweden" with Diane Cilento, "Portrait Of Murder", "A Kind Of Folly" with Dame Flora Robson, "You Never Can Tell" and "Merchant Of Venice" with Sir Ralph Richardson. In 1969 he emigrated to the United States, working in top regional theaters, off-Broadway and Broadway. His Broadway credits include the role of Col. Pickering in Rex Harrison revival of "My Fair Lady", and Ingrid Bergman's husband in "The Constant Wife" directed by Sir John Gielgud. Other Broadway shows included "Ari", "MacBeth" with Christopher Plummer and 'Glenda Jackson (I)', "Romeo and Juliet", "Lost in the Stars" with Brock Peters, "The Iceman Cometh" with James Earl Jones and with the Old Vic "MacBeth", "Romeo and Juliet" with Claire Bloom, "Richard II" and "Troilus and Cressida." He did numerous tours in the U.S including "Laurette" with Judy Holliday.

On stage he has worked with such actors as Sir Ralph Richardson, Sir Anthony Quayle, Dame Judith Anderson, Judi Dench, Sir Michael Redgrave and Richard Burton. He is also known for war films such as Pursuit of the Graf Spee (1956) with Peter Finch, The One That Got Away (1957), North West Frontier (1959) with Lauren Bacall and Sink the Bismarck! (1960). His work in epics includes Solomon and Sheba (1959) with Yul Brynner, Cromwell (1970) with Richard Harris, Oscar-winning films Lawrence of Arabia (1962), Patton (1970) and A Man for All Seasons (1966). His work in cult favorites includes Jason and the Argonauts (1963) and Clash of the Titans (1981) with Sir Laurence Olivier. His work in "Hammer" films includes Sword of Sherwood Forest (1960) with Peter Cushing, Circus of Horrors (1960) and The Curse of the Mummy's Tomb (1964). He also co-starred in the Disney hit In Search of the Castaways (1962) with Maurice Chevalier and Hayley Mills.

His voice work can be heard on Caedmon Records in "Anthony & Cleopatra", "King John", "Macbeth", "Richard II", "Titus Andronicus", "Troilus and Cressida", "Hamlet", "A Midsummer Night's Dream", and "Henry IV" parts I and II and "Henry V".