As a child, he studied math with his father, a teacher, and music with his mother. Ansermet's early training seemed to add up to a career in mathematics; he specialized in that subject at Lausanne University, graduating in 1903. Ansermet served as a professor of mathematics from 1905-1909. But during this time his interest in music only increased; he kept an eye trained on the technique of local conductors, and took courses in music with Alexandre Denéréaz, Otto Barblan, and Ernest Bloch. Ansermet sought further advice on conducting from Felix Mottl in Munich and Artur Nikisch in Berlin, then concentrated mainly on teaching himself the art of the baton. His first professional efforts were leading the summer Kursaal concerts in Montreux (1912-1914), and conducting symphonic concerts in Geneva (1915-1918). In 1918 he organized the Orchestre de la Suisse Romande in Geneva, from the start performing a substantial amount of contemporary French and Russian music. Ansermet befriended many of the great progressive composers of the time, especially Claude Debussy, Maurice Ravel, and Igor Stravinsky. Through Stravinsky, Ansermet met Serge Diaghilev and was appointed principal conductor of the latter's Ballets Russes, touring with the company to Paris, London, Italy, Spain, South America, and the United States. During a 1916 tour Ansermet made his first recordings with the Ballets Russes orchestra -- the beginning of a half century of making intriguing records with less-than-stellar ensembles. Through his association with the Ballets Russes, Ansermet was able to premiere many of the period's most important dance scores, including Falla's Three-Cornered Hat, Prokofiev's The Buffoon, Satie's Parade, and Stravinsky's Pulcinella. As an extra-curricular wartime diversion, on September 28, 1918, Ansermet premiered Stravinsky's L'Histoire du soldat in Geneva.
He developed the reputation of -- in the words of Nicholas Slonimsky -- "a scholarly and progressive musician capable of fine interpretations of both classical and modern works." Although the Suisse Romande Orchestra, with which he recorded for Decca in the 1950s and 1960s, could be criticized for its wiry strings and sour woodwinds, the group delivered to Ansermet highly accurate performances notable for their clear textures and delicate timbral balances.
Ansermet was, not surprisingly, a gifted conductor of Classical-era music, but he had little opportunity to record it. He is best remembered for his sui generis recordings of the music of his French contemporaries Debussy, Ravel, and Roussel, and his Swiss compatriots Honegger and Martin. But Ansermet was also a strong champion of such other contemporary composers as Bartók and Britten, premiering the latter's opera The Rape of Lucretia. He retired from conducting in 1967, to the end performing and committing to disc such rarities as Magnard's Symphony No. 4. Ansermet's compositions include a symphonic poem, Feuilles de printemps; he also orchestrated Debussy's Six épigraphes antiques, among other pieces. His publications include Le Geste du chef d'orchestre (1943) and Les Fondements de la musique dans la conscience humaine (1961), in which he used mathematics to discredit 12-tone and other advanced compositional techniques.