From 1921 to 1955 Harris was Professor of Organ and Harmony . He was organist at St Augustine's Church, Edgbaston from 1911 to 1919 and concurrently of Assistant Organist at Lichfield Cathedral followed in 1919 by becoming Organist successively at New College and in 1929 Christ Church, Oxford, moving to St. George's Chapel, Windsor in 1933.
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. Every Monday he would direct madrigal practise in the Red Drawing Room at Windsor, where the two Princesses sang alongside four of the senior choristers with the lower voices augmented by Etonians, Grenadier Guards and members of the Windsor and Eton Choral Society. Jars of Argentinian honey, sent to Windsor by overseas subjects, were donated by the Princesses to the Choir School as a treat for the choristers.