When World War II began, the family returned to the southwest of France. Gréco was a student at the Institut Royal d'éducation Sainte Jeanne d'Arc in Montauban. The Gréco family became active in the Résistance and her mother was arrested in 1943. The two sisters decided to move back to Paris but were captured and tortured by the Gestapo, then imprisoned in Fresnes Prison in September 1943. Her mother and sister were deported to Ravensbrück while Juliette, being only 16, remained in prison for several months before being released. After her release, she walked the eight miles back to Paris to retrieve her belongings from the Gestapo headquarters. Her former French teacher and her mother's friend, Hélène Duc, decided to take care of her.