[14] In the following years, Petri became a steady collaborator on De Santis' films, as a researcher for Rome 11:00 (1952),[14] and as an assistant director and co-writer from A Husband for Anna (1953) until La garçonnière (1960).

[14][15] In addition, Petri wrote scripts for Giuliano Puccini, Aglauco Casadio and Carlo Lizzani during this period,[13][14] and directed two documentary shorts, Nasce un campione (1954) and I sette contadini (1957).

[14][15] Petri made his feature film debut as a director with The Assassin (also titled The Lady Killer of Rome, 1961),[1] starring Marcello Mastroianni as an egotistical social careerist accused of the murder of his former mistress.

[7] We Still Kill the Old Way (1967), a crime drama following a murder investigation hindered by local power structures in rural Sicily, was adapted from the novel To Each His Own by Leonardo Sciascia and received the Best Screenplay Award at the 1967 Cannes Film Festival.

A Quiet Place in the Country (1968), a giallo thriller about an artist's deterioration into madness, won a Silver Bear award at the 19th Berlin International Film Festival,[10] but was also dismissed by some critics as "kitsch"[14] and "nonsense".

[4] The former film follows a factory worker who sides with political radicals and slowly loses his mind when he is no longer needed, the latter focusses on a bank clerk who quits his job and turns to robbery.

[4] In 1978, Petri directed Le mani sporche, a three-part television production of Jean-Paul Sartre's play Dirty Hands starring Marcello Mastroianni.