Ansermet was born in Vevey, Switzerland. Originally he was a mathematics professor, teaching at the University of Lausanne. He began conducting at the Casino in Montreux in 1912, and from 1915 to 1923 was the conductor for Diaghilev's Ballets Russes. Travelling in France for this, he met both Claude Debussy and Maurice Ravel, and consulted them on the performance of their works. During World War I, he met Igor Stravinsky, who was exiled in Switzerland and from this meeting began the conductor's lifelong association with Russian music.
In 1918 Ansermet founded his own orchestra, the Orchestre de la Suisse Romande (OSR). He toured widely in Europe and America and became famous for accurate performances of difficult modern music, making first recordings of works such as Stravinsky's Capriccio with the composer as soloist. Ansermet was one of the first in the field of classical music to take jazz seriously, and in 1919 he wrote an article praising Sidney Bechet.
After World War II, Ansermet and his orchestra rose to international prominence through a long-term contract with Decca Records. From that time until his death, he recorded most of his repertoire, often two or three times. His interpretations were widely regarded as admirably clear and authoritative, though the orchestral playing did not always reach the highest international standards, and they differed notably from those of other famous 20th-century specialists, notably Pierre Monteux and Stravinsky himself. Ansermet disapproved of Stravinsky's practice of revising his works, and always played the original versions. Although famous for performing much modern music by other composers such as Arthur Honegger and Frank Martin, he avoided altogether the music of Arnold Schoenberg and his associates, even criticizing Stravinsky when he began to use twelve-tone techniques in his compositions. In Ansermet's book, Les fondements de la musique dans la conscience humaine (1961), he sought to prove, using Husserlian phenomenology and partly his own mathematical studies, that Schoenberg's idiom was false and irrational. He labeled it a "Jewish idea" and went on to say that "the Jew is a me who speaks as though he were an I," that the Jew "suffers from thoughts doubly misformed", thus making him "suitable for the handling of money", and sums up with the statement that "historic creation of Western music" would have developed just as well "without the Jew". Ansermet's reputation suffered after the war because of his collaboration with the Nazis and he was boycotted in the new state of Israel.
In May 1954 Decca recorded Ansermet and the orchestra in Europe's first commercial stereophonic recordings. They went on to record the first stereo performance of the complete The Nutcracker by Tchaikovsky on LP (Artur Rodzi?ski had already recorded a stereo performance on magnetic tape, but this had been released on LP only in mono). Ansermet also conducted early stereo recordings of Debussy's Nocturnes and the Prélude à l'après-midi d'un faune. Part of his recording of The Rite of Spring, augmented by a rehearsal recording unobtainable elsewhere, was used by Decca on the company's 1957 stereo demonstration LP, A Journey into Stereo Sound. The conductor's clear and methodical counting of beats is a distinct feature of this rehearsal sequence. In his last years he and his ensemble recorded works by Haydn, Beethoven and Brahms.
His last recording, of Stravinsky's The Firebird, was made in London with the New Philharmonia Orchestra in 1968, which included a recording of the rehearsal sessions issued as a memorial to him. Another late recording for Decca, also issued as a memorial album, was with L'Orchestre de la Suisse Romande, and was devoted to Albéric Magnard's Symphony No. 3 and Édouard Lalo's Scherzo for Orchestra.
Ansermet composed some piano pieces and compositions for orchestra, among them a symphonic poem entitled Feuilles de Printemps (Leaves of Spring). He also orchestrated Debussy's Six épigraphes antiques in 1939.
Ernest Ansermet, 85, who founded l'Orchestre de la Suisse Romande in 1918 and conducted it with precision and puissance for 49 years; of a heart attack; in Geneva. A daring innovator, Ansermet was acclaimed by critics for his imaginative forays into Ravel, Debussy and Stravinsky. The Suisse Romande was always his first love, but he also helped found l'Orchestre Symphonique de Paris and occasionally shared the baton with Toscanini at the New York Philharmonic.