Copland's career was launched on his return to the United States in 1924, when Serge Koussevitsky agreed to conduct the Boston Symphony Orchestra in a performance of his Organ Concerto, with Boulanger as soloist. In his Music for the Theater (1925) and Piano Concerto (1927), Copland worked with jazz rhythms and Stravinsky's neoclassicism is evident in his Piano Variations (1930). Later, however, Copland developed an easier, vernacular style, using simple harmonies, folk melodies and straightforward orchestration. First apparent in his El Salon Mexico (1936), Copland's more accessible music was further developed in such distinctly American themes as Lincoln Portrait (1942), for orchestra and narrator, and the ballets Billy the Kid (1938) and Rodeo (1942).

Appalachian Spring (1944), which won the Pulitzer Prize in 1945, adapted American folk music, styles and rhythms to give a taste of early American life. It achieved both popular success and critical acclaim, although some of his contemporaries did not agree with his insistence on "saying it in the simplest possible terms". The best known of Copland's orchestral works in this style is Fanfare for the Common Man (1942).

Copland also wrote music for films, including Of Mice and Men (1937), Our Town (1940) and The Heiress (1949), which won an Oscar for Best Dramatic Film Score. By the mid 1950s, he began to turn away from the easy style that had proved so successful in the preceding decades, a departure that began with the complex Fantasy (1957) for piano. In Connotations (1962), which was commissioned for the opening of New York's Lincoln Center, he used the 12-tone system of composition initiated by Arnold Schonberg, while the symphonic Inscape (1967) is regarded as the definitive statement of his mature, "difficult" style.