Quilley was born in Islington, North London. He attended Bancroft's School in Woodford Green, London. One of his best-known roles was as Commander Traynor in the children's science fiction TV series Timeslip. He was also heard in many television voiceovers.
He had long runs on London's West End during the 1950s in Wild Thyme and Grab Me a Gondola. In the 1970s he appeared with the Royal National Theatre in Macbeth, Hamlet, The Tempest and Long Day's Journey into Night, alongside Laurence Olivier in the last. He starred as Charles Condomine in the hit show High Spirits, a successful musical version of Noël Coward's Blithe Spirit.
In 1980, he played the title role in Stephen Sondheim's Grand Guignol musical Sweeney Todd and in 1982 he played Terri Dennis in Peter Nichols' play, Privates on Parade (he appeared in the film version of Privates on Parade as well). This camp performance was repeated in 1985 in La Cage Aux Folles in which he starred with George Hearn. In 1983 he played Jupiter/Napoleon III in the BBC television production of Orpheus in the Underworld.
He appeared in two film versions of Agatha Christie's classic mystery novels: as Captain Kenneth Marshall in Evil Under the Sun and as Antonio Foscarelli in Murder on the Orient Express. He returned to the works of Noël Coward in the BBC version of Tonight at 8:30 appearing as Jasper in the Family Album playlet.
One of his last stage performances was as Elisha Whitney in Cole Porter's Anything Goes at the Theatre Royal, Drury Lane, but was too unwell to make the transfer from the National Theatre to the West End.
Denis Quilley was appointed an Officer (OBE) of the Order of the British Empire in the 2002 New Year Honours.
- Life at the Top (1965)
- Anne of the Thousand Days (1969)
- The Black Windmill (1974)
- Murder on the Orient Express (1974)
- Evil Under the Sun (1982)
- Privates on Parade (1982 film)
- A.D. (1984)
- Foreign Body (1986)
- Denis Quilley at Find a Grave
- Denis Quilley at the Internet Movie Database
- Denis Quilley at the TCM Movie Database
- Denis Quilley at the Internet Broadway Database
- Guardian obituary