Some of Copland's early songs show the influence of European music. In the impressionist "Night," we can hear Claude Debussy with his whole-tone scales, while Richard Strauss seems to be the model for "My Heart is in the East."

Through his activity at the Yaddo artist colony in Saratoga Springs (NY), Copland influenced the careers of other young American artists, including such important figures as Virgil Thomson, Leonard Bernstein, Philip Guston, Truman Capote, and Philip Roth. He also formed a group called the "commando unit," which included fellow composers Roger Sessions, Roy Harris, Virgil Thomson, and Walter Piston. The group collaborated in joint concerts that exposed their work to new audiences. Ironically, this most American of American composers was put on trial in the 1950s for allegedly not being American enough. Senator Joseph McCarthy, in his campaign against perceived Communist infiltration of the United States, questioned Copland's participation in cross-cultural exchanges with Soviet composers such as Shostakovich. Though just eight years earlier he had received the Pulitzer Prize for his ballet Appalachian Spring, Copland was ordered to appear before the House Un-American Activities Committee. As a result, for several years Copland found it difficult to obtain a passport to travel and was shunned by some public personalities. Gradually he was restored to the good graces of Congress and general opinion, and in 1986 he received the Congressional Gold Medal for his "uniquely American music that reflects the very soul and experience of our people."

Appalachian Spring premiered in 1944 at the Library of Congress in the Coolidge Auditorium. The work had been commissioned by the foundation of Elizabeth Sprague Coolidge (1864–1953), one of the most significant music patrons in America. Copland's ballet was scored for just 13 instruments—flute, clarinet, bassoon, piano, and strings—because no more would fit in the orchestra pit. The "mother of modern dance," Martha Graham, choreographed the ballet and danced the lead role.

In fact, at first Copland's had titled the piece Ballet for Martha. The subject matter evolved along with the choreography, and the title was only applied (by Graham) at the last moment. Appalachian Spring became the story of a pioneer celebration in Pennsylvania in the early 1800s. In the narrative, a farmhouse has been built for a young couple, and neighbors and a revivalist preacher offer their good wishes and advice. The music features a square dance, hymn-like melodies, and the now well-known Shaker song "Simple Gifts."

During the 30s, 40s, and 50s, inspired by the ideals of German Gebrauchsmusik ("utility music") Copland created American "music for use." The aim was to create works that served utilitarian as well as artistic purposes and had wide public appeal. Aside from Rodeo (1942), and Appalachian Spring (1944) mentioned above, music from this period includes incidental music for special occasions, plays, movies, and radio, as well as his best-known ballet scores: El Salón Mexico (1936) and Billy the Kid (1939). These works were to American music much what Stravinsky's had been for Russian music.

From the 1960s until the end of his life, Copland concentrated on conducting and recording rather than composing. "It was exactly as if someone had simply turned off a faucet," he famously remarked, referring to his creative powers. Over the course of an exceptionally active career, Copland's achievements won him all kinds of awards in the U.S. and abroad, from the Pulitzer Prize in 1945 to the Order of Merit of the Federal Republic of Germany in 1970. He died of Alzheimer's disease and respiratory failure on December 2, 1990 in North Tarrytown, New York (now Sleepy Hollow). Much of his large estate was bequeathed to the creation of the Aaron Copland Fund for Composers, which bestows over $600,000 per year to performing groups.