Deller was born in Margate, and as a boy studyied singing with his father. When his voice broke, he continued singing as a countertenor. He concentrated on English baroque and renaissance music by composers such as John Dowland and Henry Purcell. He formed the Deller Consort in 1948, a group dedicated to historically accurate performances of that kind of music.
In 1960, Deller sang the role of Oberon in Benjamin Britten's opera A Midsummer Night's Dream. This was the first role for a countertenor since the baroque period, and Britten wrote it with Deller specifically in mind.