The daughter of a schoolteacher, she was only six years old when she was cast by director René Clément to star in his film, Forbidden Games. Fossey's performance of an innocent child orphaned by World War II was widely praised by critics and audiences alike. The film won numerous awards world-wide including the Academy Award for Best Foreign Language Film and Fossey was hired by American actor/director Gene Kelly for his 1957 film, The Happy Road. She was just ten years old when filming finished and her parents decided to take her out of the film business so she could receive proper schooling and live a normal childhood.

While completing her education, Fossey studied piano and dance but went on to work in Geneva, Switzerland as an interpreter/translator. In 1967, at age twenty, she was offered the female lead by director Jean-Gabriel Albicocco for his film Le Grand Meaulnes. As a young lady, she came across on screen with the same sweet innocence that had captivated audiences as a six-year-old child in Forbidden Games. This first adult role led to a long and successful career in acting both on stage and in film, working with notable French directors such as François Truffaut and Bertrand Blier. Fluent in English, Fossey made several Hollywood motion pictures including in a 1979 role as the wife of Paul Newman in the Robert Altman directed film, Quintet. During the 1990s, she began performing almost exclusively in television productions.

Brigitte Fossey has a daughter from her marriage to director Jean-François Adam whom she met while making his 1970 film M comme Mathieu.