Another facet of his scholarship was the cataloging and analysis of the folk songs of his native country Hungary. Over many years, with his colleague Béla Bártok, he set the cornerstone for building the discipline of ethnomusicology. As a composer he is remembered chiefly for his instrumental music, the orchestral suite from his opera Háry.János, several orchestrated dance suites, the nationalistic Psalmus Hungaricus, and the universal Missa brevis: in tempore belli [in time of war] (1943/1944).

The son of a rural train station-master who also played the violin in a chamber music group, Kodály was exposed to two worlds of music from early childhood. Typical of so many composers, he also sang in a cathedral choir; but he also heard much folk music played by gypsy orchestras. While still a boy he composed the Ave Maria for soloist and orchestra, which he transcribed for female choir in 1935. Attending both the Budapest University and Budapest Music Academy, he was widely read, both in science and the arts. He did not travel outside Hungary for many years, but in 1907 he did go to France where he delighted in the music of Claude Debussy.

Kodály taught composition at the Academy after graduation, and later became the Assistant Director after the 1919 revolution. When the republic collapsed in 1921, he became persona non grata, and began concentrating on the musical education of children through an independent "Singing Youth" network throughout Hungary. He gradually regained his status as Háry.János and the orchestral Peacock Variations became worldwide successes. He began to travel throughout Europe and to the United States, conducting and receiving many awards and honorary doctorates. Though he didn't actually quote specific folk tunes, his music was nationalistic, based on Hungarian folk scales and melodic styles. His was the leading musical voice in Hungary.

During the Nazi occupation Kodály was told to divorce his Jewish wife, but he refused. Instead, he and his wife worked passionately in the resistance to help Jews escape. They hid in a suburban convent and then in the Budapest Opera House, where he composed the Missa Brevis, It was premiered in a cloakroom used as a concert room for the performance, accompanied by a small organ. In 1948 he orchestrated the Mass for a British choral festival. One wonders if he consciously turned away from his previous nationalistic way of composing towards a more universal style as lie looked forward to a more international performing career after the war.

Throughout his life Kodály taught that the foundation of all music is song, and the Missa Brevis is full of memorable melodies. He created solid and balanced forms, reiterating and transforming some of his material in various movements of the same work. Kodaly's harmonic style was also unique and refreshing, based on tradition, but also using a variety of techniques ranging from the Renaissance to the 20th century. Some composers who particularly influenced his style are Palestrina, Handel, and Debussy.

Laudes organi: Fantasia on a Twelfth-century Sequence (1966) was commissioned by the American Guild of Organists for their 1966 National Convention in Atlanta, Georgia.  It was his last completed composition, and was based on a poem praising the beauty and power of the pipe organ.