A strikingly beautiful leading lady of the Nouvelle Vague, Juliette Mayniel was the daughter of a café owner and his wife, born in the small rural community of Saint-Hippolyte, Aveyron, in southern France. After World War II, her parents resettled in Bordeaux. Juliette spent her adolescence there, went to high school, and, in due course, discovered amateur dramatics. Following a brief sojourn with a local repertory company, she moved to Saint-Germain-des-Prés in Paris, aged twenty, hoping to resume further acting studies at the Conservatoire. However, stage fright got the better of her and she failed to progress beyond the entrance exam.

For a short time, Juliette found work in the field of haute couture fashion design. Through an advertising agency, her face first popped up on screen in a commercial for a brand of soap. The director Claude Chabrol later claimed in his autobiography that this clip led him to cast Juliette as the promiscuous Florence, coveted by two law students played by Gérard Blain (as the serious Charles) and Jean-Claude Brialy (as the rakish and hedonistic Paul) in his film The Cousins (1959). For this, her first important movie role, Juliette garnered good reviews. The New York Times described her performance as "thoroughly provoking".

Her success next prompted leading roles in period drama (Pêcheur d'Islande (1959)), a crime thriller (La nuit des traqués (1959)) and in Georges Franju's classic horror film Eyes Without a Face (1960) (as the hapless recipient of a face transplant experiment). Curiously, Juliette won her one major accolade, a Silver Bear at the 10th Berlin International Film Festival, for her role in a German-produced drama film, Kirmes (1960), directed by Wolfgang Staudte. Chabrol then directed her again as one of the murder victims of France's most notorious serial killer in Bluebeard (1963), and, as Lucie, the object of a disturbed young man's fantasy in Ophélia (1963). She acted among an international cast in the 'giallo' thriller Assassination in Rome (1965) and as Priam's daughter Creusa in the peplum spectacle The Trojan Horse (1961), love interest of Trojan hero Aeneas, as portrayed by muscular American bodybuilder Steve Reeves (chiefly famous in Italy for playing Hercules).

In 1964, Juliette divorced her husband, the actor Robert Auboyneau. She was for four years the partner of well-known Italian star Vittorio Gassman with whom she had a son (actor and writer Alessandro Gassmann). She continued to act in films of widely varying quality until 1983. Juliette eventually quit show biz and lived for many years in Mexico, where she died on July 21 2023, aged 87.