[1] In the following decade, he primarily played supporting roles, with his last appearance in Federico Fellini's City of Women (1980).

While still a university student, he was spotted by director Luigi Comencini, who cast him in the lead role of his crime drama Girls Marked Danger (1952), despite his complete lack of acting experience.

He appeared mainly in Spaghetti Westerns and poliziotteschi, though he also took part in several international productions, including films by Delmer Daves (The Battle of the Villa Fiorita, 1965) and Tony Richardson (Mademoiselle, 1966).

[1] Trading on his reputation as a rugged, masculine screen hero, Manni was cast in a co-starring role by Federico Fellini as Dr. Xavier Katzone in his surreal 1980 "battle-of-the-sexes" comedy City of Women.

[2][3] He died on 27 July 1979, aged 52, at his home in Rome from blood loss caused by a self-inflicted gunshot wound to his thigh.