A skiing accident at age 16 dashed those hopes but she found a creative outlet in acting. After studying philosophy and sociology at university in Frankfurt, Germany, the Swiss-born Keller trained for the stage at Munich's Stanislavsky School and Berlin's Brecht Theatre School. She honed her craft in several German TV productions and on stage with the Heidelberg Repertory Company and the Schiller Theatre in Berlin. By 1968, Keller had moved to France where she quickly established her screen reputation in two Philippe de Broca films, "Le Diable pour le queue/The Devil By the Tail" (1968) and "Caprice de Marie/Give Her the Moon" (1969). Solidifying her reputation as a versatile and gifted performer was her turn as three generations in the same family in Claude Lelouch's "Toute une vie/And Now My Love" (1974).
Hollywood beckoned and Keller found herself cast opposite name leading men in several high profile features of the mid-decade. "Marathon Man" (1976) teamed her with Dustin Hoffman but the role wasn't much above window dressing. Far more interesting (if a little cliched) was her terrorist in John Frankenheimer's taut "Black Sunday" (1977), though she stumbled badly as Al Pacino's love interest in the misfire "Bobby Deerfield" (also 1977). Her off-screen romance with Pacino somehow did not translate before the cameras and this romantic drama set against the world of auto racing proved dull and lifeless. As a Garboesque movie star in "Fedora" (1978), Billy Wilder's "Sunset Boulevard" redux, Keller was well-cast but lacking that illusive spark that made Gloria Swanson so effective. After a turn as a spy in "The Formula" (1980), the actress gave up on Hollywood and returned to Europe.
Keller remained active on stage in France and continued to do good work in the occasional film, although few of her efforts achieved international renown. One exception was the superb "Dark Eyes" (1987), which cast her as Marcello Mastroianni's mistress. Keller also made appearances as Mrs. William Shirer (opposite Sam Waterston) in the TNT miniseries "The Nightmare Years" (1989) and replaced an ailing Lee Remick as the mother of "Young Catherine" (TNT, 1991). More recently, Keller co-starred as the childhood friend of Genevieve Bujold in the muddled "Mon Amie Max/My Friend Max" (1994) and offered an insightful cameo as a German Jewess in pre-war Europe in "According to Pereira" (1995), again opposite Mastroianni.