In 1904 he became the conductor of the Concerts Populaires at Lille, and the following year he joined with cellist Pablo Casals and violinist Jacques Thibaud to form one of the greatest of permanently established piano trios, one which became a model of its type, touring frequently. This drew him back to the piano, which he had never given up despite his fame as a conductor. In 1907 he joined the faculty of the Paris Conservatory, teaching piano, but remained very active as a piano soloist and chamber music player. He gave up that position in 1917, feeling that his busy concert schedule had made it impossible to devote sufficient uninterrupted periods to teaching. In 1919 he founded the Ecole Normale de Musique, assembling a faculty of famous musicians. As the director, he taught a summer course in interpretation, which became famous. He continued a career performing piano around the world, including lecture recitals, and also guest conducted many orchestras. He also continued to premiere new French piano music. Cortot was a skillful and scholarly editor of great piano music, famous for his editions of most of Chopin's piano music. Cortot's teacher was a student of Chopin, and the grace of his Chopin performances, especially, remains breathtaking and should be recommended to all students of the piano; he also had a remarkable way with the music of Robert Schumann.
In 1943 Cortot founded the Chamber Music Society of the Paris Conservatory Concerts. However, his admiration for German culture served him ill when Germany occupied France between from 1940 to 1944, and he appeared to cooperate with them willingly. This led to his being shunned after the war both in France and elsewhere. By the time he returned to the concert stage, some years later, it was clear that his memory was failing. His main legacy remains in the records he made in the 1920s and 1930s. In addition, he was an avid collector and amassed, among other items, a large quantity of autograph scores and printed music. After his death, this collection was divided among several important libraries and universities; it remains an interesting view into the mind of a musician who was both a living link to Romantic Paris and a key figure of the twentieth century.