Barber intensely played and studied the music of J S Bach. He also was an adherent of Brahms, from whom he learned how to compress profound emotions into small modules of highly charged musical expression (Cello Sonata, 1932). In 1933, after reading the poem Prometheus Unbound by Percy Bysshe Shelley, Barber composed the tone poem Music for a Scene from Shelley. In 1935, the work was premiered at Carnegie Hall, and this was the first time the composer heard one of his orchestral works performed publicly. Barber’s compositional style has been lauded for its musical logic, sense of architectural design, effortless melodic gift, and direct emotional appeal. These characteristics remained in his music throughout his lifetime.

His compositions would later include characteristics of polytonality (Second Symphony, 1944), atonality (Medea, 1946; Prayers of Kierkegaard, 1954), Twelve-tone technique (Nocturne, 1959 and the Piano Sonata, 1949), and even jazz (Excursions, 1944; A Hand of Bridge, 1959). Barber's composition were never lauded to be pathbreaking, but his compositions were an eclectic blend of the “musical currents hovering about in his time”. John Corigliano succinctly described Barber's style as "an interesting dichotomy of harmonic procedures—an alternation between post-Straussian chromaticism and often diatonic typical American simplicity."

With a background deeply rooted in vocals, Barber's love of poetry and his intimate knowledge and appreciation of the human voice inspired his vocal writing. Barber's most famous vocal compositions, Knoxville: Summer of 1915 (to words by James Agee) and Dover Beach (to words from a Victorian text by Matthew Arnold), were greatly successful and received critical acclaim, making a powerful case for Barber as one of the twentieth century's most accomplished composers for the voice.