Born in 1923 in Antony, Hauts-de-Seine, France, Lebeau began her career in film immediately before the start of World War II with the 1939 French drama Jeunes filles en détresse (Girls in Distress). The following year, in a scene right out of the film for which she would become best known, Lebeau, who was Jewish, was forced to flee Paris with her then-husband Marcel Dalio ahead of the advancing German invasion. The couple ended up in Lisbon, Portugal and from there attempted to travel to the Americas on a Chilean visa. Their transit across the Atlantic was stopped in Mexico when it was discovered their visas were forgeries, but were eventually able to secure temporary Canadian passports which they used to make their way to the United States.