Keller received his dramatic training at Lee Strasberg’s prestigious Actors Studio in New York. His first stage engagement took him to Broadway. From 1968 until his departure to appear in Fellini Satyricon, he was a member of the tribe in the original line-up stage production of Hair, directed by Tom O’Horgan, written by Gerome Ragni and James Rado, music by Galt MacDermot. In addition, Keller worked as a model and acted in the New York underground scene with Andy Warhol. Two years later, Keller was in Fellini Satyricon, his feature film debut. The Italian director Federico Fellini hired him for the leading role in his gloomy-looking adaptation of the novel fragment Satyrica of Petronius as his preferred candidate for the role since Pierre Clémenti was no longer available. His role as Ascyltus helped boost Keller’s career internationally. The screenwriter and long-time employee of Fellini Bernardino Zapponi wrote of Keller’s representation: … Has a very photogenic smile and is full of knowledge as required by his role.  After this success, Keller received a number of major roles in mostly Italian productions. He starred with Giancarlo Giannini in Alberto Lattuada’s comedy Sono stato io!, the male lead role opposite Jane Birkin in the Anthony M. Dawson-directed horror movie Seven Deaths in the Cat’s Eye (1973), the main role in the science fiction thriller The Secret of Life (with Klaus Kinski), Catherine Breillat’s Une vraie jeune fille and drama in the historical film Rivuole Cesare Roma!. In 1970, he also appeared in an Italian-French-German-Bulgarian co-production of Jules Verne’s The Courier of the Czar, directed by John Phillip Law. In 1982 he starred with his wife, Kristina St. Clair the Jamaican film production Countryman.