Early years
Quilley was born in Islington, North London, the son of Clifford Charles Quilley, a Post Office telegraphist, and his wife Ada Winifred, née Stanley. He won a scholarship to Bancroft's School in Woodford Green, London, and was expected to go from there to a university, but he was determined to become an actor as soon as possible. He made his stage debut with the Birmingham Repertory Theatre Company, during the 1945 season, in a company that, he recalled, included Paul Scofield, Stanley Baker, Paul Eddington, Alun Owen and "a 20-year-old wunderkind director called Peter Brook, of whom everybody was already in some awe."
Quilley's early career was interrupted when he was conscripted for national service in the army, based in Khartoum. His first London appearance after his release from the forces was at the Globe in 1950, when he took over the part of Richard in John Gielgud's production of The Lady's Not For Burning from Richard Burton, whom he had understudied in the early months of the run. The understudy to Claire Bloom in the play was Stella Chapman, whom Quilley married in 1949. They had a son and two daughters.
Later in 1950 Quilley joined the Old Vic Company for a British Council tour of Italy, playing Fabian in Twelfth Night and Gratiano in The Merchant of Venice. He took part in a revival of the 17th century gigue Michael and Francis at Hampton Court as part of the Festival of Britain in 1951, and sang in other performances by the London Opera Club alongside established opera singers, including their fifth anniversary concert at the Arts Council in 1953. In 1953 he appeared in revue, with Max Adrian, Betty Marsden and Moyra Fraser in Airs on a Shoestring, which ran for more than 700 performances. In 1955 he had his first leading role in a West End production, playing Geoffrey Morris in the musical Wild Thyme, by Philip Guard and Donald Swann. In The Manchester Guardian, Philip Hope-Wallace wrote, "Denis Quilley turns out a comparatively rare figure nowadays: a presentable singing English hero, a most likeable performance." In 1956 Quilley opened in another long-running show, Grab Me a Gondola which played for more than 600 performances.
One of Quilley's other singing roles of the 1950s was the title character in Leonard Bernstein's operetta Candide. It ran for only sixty performances in this first London production in 1959; Quilley returned to the piece later in his career, playing the bombastic Baron and the misanthropic Martin in a National Theatre production in 1999. Quilley made no cinema films in the 1950s, but appeared in several television productions, ranging from Shakespeare (Bassanio in The Merchant of Venice, 1955) to detective fiction (Jimmy Sutane in Dancers in Mourning (1959)).