At the age of 14, he took up a "flexible" position as assistant organist at St David's Cathedral in Wales under Herbert Morris, followed at 16 by a scholarship to the Royal College of Music.

[3] There, he was at his most productive: composing for the Three Choirs Festival, conducting at both the 1937 and 1953 coronations, and producing two orchestral pieces premiered at The Proms: the overture Once Upon a Time (1940) and the Heroic Prelude (1942).

[4] Bruce Nightingale, who became senior chorister at Windsor during the wartime years, describes "Doc H" as having "a fat, usually jolly face with a few wisps of hair across an otherwise bald head."

Although choir practice was normally conducted in a "benign atmosphere," Nightingale recounts that Harris would occasionally complain of a "batey practise" and, on the rare occasions he considered a performance mediocre, would scold the choirboys in a loud stage whisper from the organ loft.

Harris was involved in the musical education of the teenage Princesses Elizabeth and Margaret Rose, who spent the wartime period at Windsor Castle.

His most famous works are two anthems for unaccompanied double choir: Faire is the heaven (1925), a setting of Edmund Spenser's poem "An Hymne of Heavenly Beautie";[8] and Bring us, O Lord God, a setting of a poem by John Donne first heard in Windsor on 29 October 1959, and which was sung at the Committal Service of Queen Elizabeth II at St George's Chapel, Windsor Castle on 19 September 2022.

His largest composition, the 1919 choral-orchestral cantata The Hound of Heaven (a setting of the religious allegory by Francis Thompson), has been almost completely forgotten.