Born as Clémence Guichard in Meudon, a southwestern suburb of Paris, the daughter of actor/writer Étienne Guichard and a French teacher, Poésy took her mother's maiden name as her stage name. She was sent to an alternative school in Meudon. Her father gave Poésy her first acting job when she was a child. She has a younger sister, Maëlle Poésy-Guichard, who is also an actress.