Sowerby began piano lessons when he was seven and was teaching himself theory from a textbook at 11. He moved to Chicago in 1909 and continued his music studies while at Englewood High School. At age 15, Sowerby received some cursory instruction on the organ, but from then on would be self-taught. Sowerby's debut as a composer came in 1913 when the Chicago Symphony at an all-America concert performed his Concerto for Violin and Orchestra. Four years later, Sowerby gave his first public performance on the piano at the Norfolk, Connecticut, Festival. His first pair of published compositions was a Woodwind Quintet (1916) and Serenade for string quartet (1917), which was a birthday present to Elizabeth Sprague Coolidge. In December 1917, Sowerby went into the U.S. Army and served for 15 months in England and France as a clarinetist and bandmaster. Sowerby remained in Europe after his discharge from the army and began composing a series of works including his First Symphony, which led to him being awarded the first American Prix de Rome in 1921. For the next three years Sowerby resided at the American Academy in Rome, writing among others From the Northland a composition that would win the Society for the Publication of American Music Award. Sowerby returned to the U.S. in 1924 and began writing music embracing American idioms including two pieces for jazz band, Syncopata and Monotony. Sowerby even adapted the popular folk tune "Pop Goes the Weasel" and arranged it for flute, oboe, clarinet, and oboe. In 1932, Sowerby joined the faculty of the American Conservatory in its composition department and remained there until 1962. He also served as choirmaster and organist of St. James Episcopal Cathedral in Chicago from 1927 to 1961, where he wrote a large body of music for chorus and organ. His love of religious music led to his founding of the College of Church Musicians in Washington D.C., which he served as director until 1968. Prior to World War II, Sowerby was one of the most frequently performed American composers and by 1943, had received the Society for the Publication of American Music Award four times. He received the Pulitzer Prize for music in 1946 for his The Canticle of the Sun, a cantata for chorus and orchestra, which employed Matthew Arnold's English translation of the canticle of St. Francis of Assisi. Later awards include an honorary fellowship at Trinity College in London and the Royal School of Church Music in Croydon, England (1963). In the same year, Sowerby was presented to Queen Elizabeth II.