Born in Paris, from an unknown father and Rose Clémenti, a Corsican concierge whose surname he held from. He had a difficult childhood and took refuge in literature and the theatre. Clémenti studied drama and began his acting career in the theatre. He secured his first minor screen roles in 1960 in Yves Allégret's Chien de pique performing alongside Eddie Constantine. Arguably, his most famous role was that of gangster lover of bourgeois prostitute Catherine Deneuve in Belle de jour, the 1967 classic by Luis Buñuel, in whose film The Milky Way he played the Devil. He appeared in several highly regarded films of the period, working with many of Europe's best known Directors, including Luchino Visconti (The Leopard), Pier Paolo Pasolini (Pigsty) and Bernardo Bertolucci (The Conformist and Partner). Other Directors he has worked with are Liliana Cavani, Glauber Rocha, Miklós Jancsó, Jacques Rivette, and Philippe Garrel.