Sumsion was born in Gloucester, a cathedral city on the River Severn. In 1908, at the age of nine, Sumsion became a probationer in the Gloucester cathedral choir, which was then under the direction of Herbert Brewer. After two years Sumsion became a full chorister and sang with the choir until 1914. From 1911 to 1915 he was organist at Ashleworth, a small village six miles from Gloucester, responsible for the church choir and the "organ blower" for a fee of £5 a quarter, quite a responsibility for a 12-year-old. Gloucester was (and is) one of the three host cities, along with Worcester and Hereford, of the Three Choirs Festival, an annual festival of choral emphasis first held in the early 18th century. Sumsion would later write of his musically formative experiences at the cathedral: 'Quite soon after my entry into the choir I was singing with the Choirs Festival Chorus and gradually absorbing the choral music of the great classical composers and the contemporary writers, of whom the giant was certainly Elgar.' When Sumsion's treble voice broke at age 15, he became an 'articled pupil' to Brewer, a designation connoting a three-year apprenticeship in organ, choral direction, and music theory. As one of Brewer's articled pupils Sumsion was following in the footsteps of his slightly older contemporaries, Herbert Howells and Ivor Gurney. Sumsion passed the Associateship exam of the Royal College of Organists in 1915, and in July 1916 joined Howells in passing the Fellowship exam; though he was only 17, Sumsion was awarded the Turpin prize for the second-highest marks in the practical component.