That same year, Hanson got his first full-time position, as a music theory and composition teacher at the College of the Pacific in California, and only three years later, the College appointed him Dean of the Conservatory of Fine Arts in 1919. In 1920 Hanson composed The California Forest Play, his earliest work to receive national attention. Hanson also wrote a number of orchestral and chamber works during his years in California, including Concerto da Camera, Symphonic Legend, Symphonic Rhapsody, various solo piano works, such as Two Yuletide Pieces, and the Scandinavian Suite, which celebrated his Lutheran and Scandinavian heritage.
Hanson was the first recipient of the Prix de Rome, awarded by the American Academy in Rome, in 1921, for both The California Forest Play and his symphonic poem Before the Dawn. Thanks to the award, Hanson lived in Italy for three years. During his time in Italy, Hanson wrote a Quartet in One Movement, Lux aeterna, The Lament for Beowulf, and his Symphony No. 1, "Nordic", the premiere of which he conducted with the Augusteo Orchestra on May 30, 1923. Ottorino Respighi, who studied orchestration with Nikolay Rimsky-Korsakov, taught orchestration to Hanson in Rome.
Upon returning from Rome, Hanson's conducting career took off, making his premiere conducting the New York Symphony Orchestra in his tone poem North and West. In Rochester, New York in 1924, he conducted his Symphony No. 1, and this brought him to the attention of George Eastman, inventor of the Kodak camera and roll film, who chose Hanson to be director of the Eastman School of Music. Hanson held that position for forty years, turning the institution into one of the most prestigious music schools in America. He accomplished this by improving the curriculum, bringing in better teachers and refining the school's orchestras. Also, he balanced the school's faculty between American and European teachers, even when this meant passing up Bela Bartok.
In 1925, Hanson established the American Composers Orchestral Concerts. Later, he founded the Eastman-Rochester Symphony Orchestra, which consisted of first chair players from the Rochester Philharmonic Orchestra and selected students from the Eastman School, and then The Festivals of American Music followed. Hanson made many recordings with the Eastman-Rochester Symphony Orchestra, not only his own works, but also those of other American composers such as John Alden Carpenter, Charles Tomlinson Griffes, John Knowles Paine, Walter Piston, William Grant Still, and other, lesser known, composers. Hanson estimated that over 2000 works by over 500 American composers were premiered during his tenure at Eastman.
To celebrate the fiftieth anniversary of the Boston Symphony Orchestra, Serge Koussevitzky commissioned Hanson's Symphony No. 2, the "Romantic", and premiered it on November 28, 1930. This work was to become Hanson's best known.
Hanson's opera Merry Mount is credited as the first American opera, since it was written by an American composer and an American librettist on an American story, and it was premiered with a mostly American cast at the Metropolitan Opera in New York, in 1934.
Hanson was elected as a member of the National Institute of Arts and Letters in 1935, President of the Music Teachers' National Association from 1929 to 1930, and President of the National Association of Schools of Music from 1935 to 1939.
Hanson and Walter Piston were part of the committee that awarded the 1941 Prix de Rome to Harold Shapero.
After he composed the Hymn of the Pioneers to celebrate the 300th anniversary of the first Swedish settlement in Delaware, Hanson was selected as a Fellow of the Royal Swedish Academy in 1938.
In 1944 Hanson was awarded the Pulitzer Prize for Symphony No. 4.
Hanson met Margaret Elizabeth Nelson at her parents' summer home on Lake Chautauqua near the Chautauqua Institution in New York. Hanson dedicated the Serenade for Flute, Harp, and Strings, to her. They married on July 24, 1946 at her parents' summer home.
In 1946, Hanson was awarded the George Foster Peabody Award "for outstanding entertainment programming" for a series he presented on the Rochester, New York radio station WHAM in 1945.
From 1946 to 1962 Hanson was active in UNESCO . UNESCO commissioned Hanson's Pastorale for Oboe and Piano, and Pastorale for Oboe, Strings, and Harp, for the 1949 Paris conference of the world body.
In 1953 Hanson helped established the Edward B. Benjamin Prize "for calming and uplifting music" written by Eastman students. Each submitted score was read by Hanson and the Eastman Orchestra. Winners of the Benjamin Prize appeared on Hanson's recording Music for Quiet Listening.
Frederick Fennell, conductor of the Eastman Wind Ensemble, described Hanson's first band composition, the 1954 Chorale and Alleluia as "the most awaited piece of music to be written for the wind band in my twenty years as a conductor in this field." Chorale and Alleluia is still a required competition piece for high school bands in the New York State School Music Association's repertoire list and is one of Hanson's most frequently recorded works.
In 1960 Hanson published Harmonic Materials of Modern Music: Resources of the Tempered Scale, a book that would lay the foundation for pitch class set theory.
Hanson took the Eastman Philharmonia, a student ensemble, on an European tour from 1961 to 1962, which passed through Paris, Cairo, Moscow, and Vienna, among other cities.
Hanson was on the Board of Directors of the Music Educators National Conference from 1960 to 1964.
Even after his retirement from Eastman in 1964, Hanson continued his association with the school.
Hanson's Song of Democracy, on a Walt Whitman text, was also performed at the inaugural concert for incoming U.S. President Richard Nixon in 1969, an event Hanson proudly described as the first inaugural concert featuring only American music.
The Eastman Kodak company, in recognition of Hanson's achievements, donated $100,000 worth of stock to the school in 1976. Hanson stipulated that the gift be used to fund the Institute of American Music at Eastman.
Hanson continued conducting in his eighties, up to his death on February 26, 1981 in Rochester, New York.