He was one of the last American composers to develop a distinctive mode of expression based wholly on the principles and techniques of late romanticism.
In 1955, he won a Fulbright Fellowship to study at the Accademia Nazionale di Santa Cecilia in Rome where he worked under Ildebrando Pizzetti.
As a composer, Flagello held firmly to a belief in music as a personal medium for emotional and spiritual expression.
He produced a large body of work, including seven operas, two symphonies, eight concertos, and numerous orchestral, choral, chamber, and vocal compositions.
In the mid-1980s, his career was cut short by a degenerative disease, at which time a number of his works were left without orchestration.