In 1985, when Testud was 14, she started taking acting classes in Lyon with the actor and director Christian Taponard. In 1989, she moved to Paris and studied for three years at the Conservatoire (CNSAD). In the early and mid 1990s, she began to have small roles in films. She acted in "L'Histoire du garcon qui voulait qu'on l'embrasse" (directed by Philippe Harel) and "Love, etc." (directed by Marion Vernoux). In 1997 she had an important role in Caroline Link's "Jenseits der Stille". This film was a great success in Germany. She had to learn German, sign language, and the clarinet for this film. In 1998, she played Béa in Thomas Vincent's "Karnaval". This was her first important role in a French film. In 2000, she performed in Chantal Akerman's "La Captive". In 2001 she won the César Award for Most Promising Actress for her role in "Les Blessures assassines" (English title: "Murderous Maids").