From 1952 to 1969 he was a cellist with the Vienna Symphony Orchestra. He soon became interested in performing old music with period instruments. He started a group called Concentus Musicus Wien. They tried to make the music of composers from the Baroque period sound like it had done when it was written. Harnoncourt played the viola da gamba as well as the cello. In 1970 he conducted Monteverdi's opera "Ritorno d'Ulisse in Patria" at La Scala, Milan.