After suffering a serious back injury, he was obliged to return for treatment to the United States, where Harris formed associations with Howard Hanson at the Eastman School of Music in Rochester and, more importantly, with Serge Koussevitsky at the Boston Symphony Orchestra. These associations secured performance outlets for the large-scale works he was writing. In 1934, a week after its première under Koussevitsky, his Symphony ‘1933’ became the first American symphony to be commercially recorded. It was his Symphony No.3, however, premièred by Koussevitsky in 1939, which proved to be the composer's biggest breakthrough and made him practically a household name.
During the 1930s Harris taught at Mills College—where Darius Milhaud would later teach—Westminster Choir College (1934-1938) and the Juilliard School of Music; he spent most of the rest of his professional career restlessly moving through teaching posts and residences at colleges and universities in various parts of the USA, ending with a long stint in California, first at UCLA and finally at California State University, Los Angeles. Among his pupils were William Schuman, H. Owen Reed, John Donald Robb, John Verrall, and Peter Schickele (best known as the creator of P. D. Q. Bach). He received many of America's most prestigious cultural awards, and at the end of his life was proclaimed Honorary Composer Laureate of the State of California.
Harris was a champion of many causes (he founded the International String Congress to combat what was perceived as a shortage of string players in the U.S., and co-founded the American Composers Alliance), a tireless organizer of conferences and contemporary music festivals, and a frequent radio broadcaster. He made several trips to the Soviet Union; his admiration for that country attracted adverse criticism during the McCarthy era. Harris was indeed in US terms a liberal on many social issues, and pugnaciously opposed to anti-semitism and racial discrimination. His last symphony, a commission for the American Bicentennial in 1976, was mauled by the critics at its first performance as a 'travesty of music' by a composer who had written himself out: but this may partly have been because it addressed the themes of slavery and the Civil War, and contradicted the required mood of national self-congratulation. In his last years Harris was increasingly depressed by the effects of the US's rampant materialism, discrimination against minorities and destruction of natural resources.
Although the rugged American patriotism of his works of the 1930s and 1940s is reflected in his research into and use of folk-music (and to a lesser extent of jazz rhythms), Harris was paradoxically obsessed with the great European pre-classical forms, especially the monolithic ones of fugue (which we hear in the Third Symphony) and passacaglia (as featured in the next most admired, the Seventh). His customary mode of discourse, with long singing lines and resonant modal harmonies, is ultimately based on his admiration for and development of Renaissance polyphony—and also antiphonal effects, which he exploits brilliantly with a large orchestra. Like many American composers of his time, he was deeply impressed by the symphonic achievement of Sibelius (who also drew on Renaissance polyphonic techniques). In Harris's best works the music grows organically from the opening bars, as if a tiny seed gives birth to an entire tree; and this is certainly the case with the Third Symphony, which joined the American repertoire during the same era as works by Aaron Copland and Virgil Thomson. The first edition of Kent Kennan's The Technique of Orchestration (1952) quotes three passages from this symphony to illustrate good orchestral writing for cello, timpani, and vibraphone, respectively. The book quotes no other Harris symphonies. Few other American symphonies have acquired such a firmly-entrenched position in the standard performance repertory as has this one, due much to the championship of the piece by Leonard Bernstein, as well as to his several recordings of it.
His music, while often abstract, has a reputation for its optimistic, American tone. Musicologist John Canarina describes the "Harris style" as "exuberant horn passages and timpani ostinatos" (Canarina 1995). Harris so frequently composed prismatically modulating chords that a valid one-word description of his orchestral music would be chromatic. He also liked to write bell-like passages for tuned percussion. This is readily apparent not only in the famous Third Symphony but also in the Sixth "Gettysburg".
In all, Harris composed over 170 works, including many works for amateurs, but the backbone of his output was his series of symphonies. Harris wrote no opera, but otherwise covered all the main genres of orchestral, vocal, choral, chamber and instrumental music as well as writing a significant number of works for band. His series of symphonies is still his most significant contribution to American music.
Harris composed at least 18 symphonies, though not all of them are numbered and not all are for orchestra. A full list is as follows:
In addition there is a missing (and perhaps not completed) Symphony for High School Orchestra (1937) and the following unfinished or fragmentary works: