From 1917 to 1919 Sumsion served in the Queen's Westminster Rifles and spent time in the Flanders trenches. In 1919 he returned to Gloucester cathedral to take up an appointment as assistant organist to Brewer. Sumsion's duties during this period included serving as accompanist for the Three Choirs Festival Chorus, which occasioned a brief but memorable encounter with Elgar after a rehearsal of "The Dream of Gerontius". Sumsion earned an undergraduate degree in music from Durham University in 1920 and continued in his post at Gloucester until 1922, when he embarked for London to become organist of Christ Church, Lancaster Gate. In 1924 he took on two additional posts—director of music at Bishop's Stortford College, north of London, and a teaching position at Morley College. He also studied conducting with Adrian Boult at the Royal College of Music, though Boult observed that Sumsion's conducting technique was already well-developed.