[1] He worked with many notable directors, including Luis Buñuel, Abel Gance, Jean Renoir, Claude Chabrol, Sidney Lumet, Joseph Losey, Chantal Akerman and Robert Altman.

Cassel gained prominence in the late 1950s as a hero in comedies by Philippe de Broca such as Male Companion and through his role as 'Jean François Jardie' in the famous French resistance piece L' Armée des ombres.

During the 1960s and 1970s, he worked with Claude Chabrol (The Breach), Luis Buñuel (as Stéphane Audran's husband in The Discreet Charm of the Bourgeoisie 1972), Ken Annakin (as the Frenchman in Those Magnificent Men in Their Flying Machines 1965), Gérard Brach (as Claude Jade's lover in The Boat on the Grass), Richard Lester (as Louis XIII of France in The Three Musketeers 1973 and its sequel The Four Musketeers 1974), Sidney Lumet (as Pierre in Murder on the Orient Express) and Joseph Losey (with Isabelle Huppert in The Trout).

In 2007, Cassel appeared in dual roles (as Père Lucien and the Lourdes souvenir vendor) in Julian Schnabel's film The Diving Bell and the Butterfly.

During his career, Cassel starred in more than 110 films, fifty stage plays, and many musical theatre performances and television shows.