Born in Canterbury, Kent, England, he served in the Royal Navy for over twenty years, attaining the rank of Commander. During his time in the Navy, he became a champion boxer and rugby player, and when he was discharged he was one of the youngest men ever to obtain the rank of Commander in the Royal Navy. Gwillim began his acting career in earnest in the 1950s, working on both stage and screen. On stage, he appeared both at the Shakespeare Memorial Theatre in Stratford and at the Old Vic. He performed in an extensive amount of theatre, both classics and modern plays, in the West End of London and on Broadway. Some of his most notable roles include: playing in Sir Ralph Richardson’s production of The Merchant of Venice; The Right Honourable Gentleman with lifelong friend Anthony Quayle; a revival of My Fair Lady with Rex Harrison, playing Colonel Pickering; John Gielgud’s The Constant Wife, with Ingrid Bergman; and The Iceman Cometh, with James Earl Jones. Gwillim also featured in over sixty films and television series, usually war films or historical epics. His military background, commanding presence and deep, booming voice typecast him as soldiers and authority figures. Some of his most notable roles include playing a warship captain in Sink the Bismarck! (1960), the archbishop Hubert Walter in Sword of Sherwood Forest (1960), the obnoxious club secretary in Lawrence of Arabia (1962), King Aeëtes in Jason and the Argonauts (1963), an RAF officer in the James Bond film Thunderball (1965), the Lord Chief Justice in A Man for All Seasons (1966), General Harold Alexander in Patton (1970), Poseidon in Clash of the Titans (1981), and Van Helsing in The Monster Squad (1987). He also had a recurring role on the TV series Danger Man, The Saint, and The Troubleshooters. He took part in a number of recordings for Caedmon Shakespeare Records.

Gwillim was twice married: to Peggy Bollard, until 1958, and Olivia Selby, from 1969 until his death. He had two children from his first marriage, Sarah-Jane Gwillim and David Gwillim, and a third, Jaxon Duff Gwillim, from his second marriage. His children also became actors, and he acted onstage with them in 1995 in a production of On Borrowed Time, which was his last on stage performance. He died in Los Angeles, California on 2 July 2001 at the age of 91.