Theodore Morrison began composing at the age of forty-two, more than twenty years after he was well established as a conductor specializing in large works for voices and orchestra, as well as music for chamber orchestra. Over the past three decades he has composed an epic choral symphony and a number of other large works. He has created a substantial body of shorter pieces including an overture for wind ensemble, chamber works for woodwinds and strings, a sonata and a set of variations for organ, several works for chorus and organ, four song cycles, and many smaller choral pieces and songs. His music has been performed throughout North America, Europe, Asia, and New Zealand.

Through his long career as a conductor, Morrison gained an intimate understanding of the voice and the orchestra, preparing him to compose for both beginning in his early forties. He founded the Baltimore Choral Arts Society in 1966 and served as its music director for sixteen years. Under his leadership, BCAS became one of the most respected independent choral/orchestral organizations in the United States. He engaged many of the finest vocal soloists of the time for large oratorio productions. These included John Aler, Helen Boatwright, Elaine Bonazzi, Charles Bressler, Phyllis Bryn-Julson, Lily Chookasian, Donald Gramm, Tom Krause, Leslie Guinn, Sherrill Milnes, John Reardon and Benita Valente. Morrison worked principally with three instrumental ensembles: The Baltimore Symphony Orchestra, which he also guest conducted on numerous occasions; The Concerto Soloists of Philadelphia (now The Chamber Orchestra of Philadelphia); and the early music ensemble Pro Musica Rara.

Morrison was director of choral music and conductor of the chamber orchestra at Peabody Conservatory of Music of The Johns Hopkins University from 1975-1978, and held a similar post at Smith College in Northampton, Massachusetts from 1981-1987. As a member of the faculty of The University of Michigan's School of Music from 1987 to 2005 he served as both director of university choirs and director of graduate studies in conducting. Morrison began his professional career at age nineteen as organist-choirmaster at Baltimore's Cathedral of the Incarnation, a post he held for ten years.