Nevertheless, as Thorne and Davies remarked in the course of their talk, it remained the case that American orchestras played relatively little American music. They decided to attempt to form an orchestra to fill that function. Davies volunteered to serve as principal conductor and music director, and another composer, Nicolas Roussakis, joined Thorne. Another conductor (also flutist) Paul Lustig Dunkel also joined the group of leaders of the endeavor. The orchestra was formed and had its first concert on February 7, 1977, which was broadcast on National Public Radio and Voice of America. The American Composers Orchestra's first several series of concerts were held at Alice Tully Hall in New York. In the 1985 - 1987 season, it permanently moved to Carnegie Hall. The immediate result of this move was that subscriptions tripled and average attendance more than doubled and the orchestra included in its activities the commissioning of new music. Joseph Schwantner's Aftertones of Infinity won the Pulitzer Prize in Music in 1978 - 1979, the first A.C.O.-commissioned work to do so. Another A.C.O. commission, Ellen Taafe Zwilich's Symphony No. 1, in 1983 became the first work by a woman to win the Pulitzer Prize.
In 1979, the American Composers Orchestra received its first A.S.C.A.P. (American Society of Composer's and Publishers, the primary composers' and publishers' rights groups in the U.S.) Award for adventuresome programming and it went on to win 18 more by the end of the twentieth century.
In 1989, the A.C.O. inaugurated an annual series of reading sessions for works of emerging composers. A reading session is a chance for a composer to hear an orchestra work on one of his/her compositions. It fulfills the difficult-to-attain goal of allowing a composer to hear in the real world whether his/her orchestration works. The orchestra encourages younger composers to send in their resumes and scores of their orchestral works and selects six or seven for playing in a two-day session. One of the composers receives a commission, based on the promise shown by the composition submitted, for a new work to be played by the A.C.O. The series was interrupted, but was reorganized on a permanent basis in 1994 with support from the Helen F. Whitaker Fund.
In the same year, the A.C.O. premiered its series of concerts on American Public Radio. In 1990, an A.C.O. recording of works by Alan Hovhanes and Lou Harrison hit the Billboard classical charts and remained there for three months. Following that, the A.C.O. signed a contract with the ARGO label, an imprint of Decca (London) and part of the major Polygram Group. Decca had decided to devote this sublabel to British, American, and choral music.
The scope of the term "American" in the ensemble's name was enlarged in the 1993 - 1994 season when its wider meaning, embracing all the nations of the New World, was recognized. The A.C.O. accordingly launched its Sonidos de las Américas (Sounds of the Americas) festival. These annual festivals are devoted to the music of individual countries or regions of the Americas. The series has included festivals focused on Mexico, Brazil, Venezuela, Puerto Rico, Argentina, and Cuba. After the 2000 - 2001 season, the series was to be re-evaluated, with a goal of establishing a broader Latin American music program the following year.
The orchestra annually commissions an average of four compositions, totaling more than 90 from 1977 to 2000. Two more of its recordings were high sellers: a disc of music by Colin McPhee, Lou Harrison, and Chinary Ung, and its best-selling disc ever, Philip Glass' Heroes Symphony. The A.C.O. has released at least 19 CDs on the Argo, ECM, Point, and Nonesuch labels. Its school music program annually plays before 20,000 New York City children in trips to several schools and in eight youth concerts at Carnegie Hall.