From age eight she studied ballet, but a skiing accident at 16 obliged her to give up dancing. She took up theater, and for three years she studied acting at Munich's Stanislavsky School. After working with a Heidelberg repertory group she eventually made it to the famous Schiller Theater in Berlin, where she performed in numerous lead roles in the classic repertoire. Meanwhile, she also appeared on German TV and made her screen debut in the British espionage film Funeral in Berlin (1966), shot on location. Later, Keller moved to France, working on both stage and TV. From 1969 she appeared in a steady string of French films, soon becoming a popular screen presence; she also became romantically involved with two of her directors, Philippe de Broca and Claude Lelouch, and she had a son by de Broca. Now established as a skilled, attractive screen actress, she was invited to Hollywood, and from 1976 (beginning with Marathon Man) she appeared in both American and international productions. She was romantically linked with actor Al Pacino, her costar in Bobby Deerfield (1977).