His three Oscar nominations for Divorzio all'italiana (1961), Giornata particolare, Una (1977), and Oci ciornie (1987) are the record for a performer in a foreign language film. The only other performers with multiple Oscar nominations for foreign language films are Sophia Loren, Liv Ullmann and Isabelle Adjani with two each.

Since 1998, a "Marcello Mastroianni Award" is given to the best "first time" young actor/actress at the Venice Film Festival.

Uncle of Federica Mastroianni.

During his studies at the Centro Universiatio Teatrale, he got to know Luchino Visconti, who gave him a role in "Un Tram che si chiama desiderio" (A Streetcar Named Desire) under his direction. At that time, he also met Anna Magnani and Federico Fellini.

Father of Chiara Mastroianni and Barbara Mastroianni

Ex-brother-in-law of French actress Françoise Dorléac, Sylvie Dorléac and Danielle Dorléac (b. 1946).

Older brother of film editor Ruggero Mastroianni, who edited several of Marcello's films directed by Federico Fellini

Told interviewers that Federico Fellini hired him for Dolce vita, La (1960) because he had a "terribly ordinary face".

From 1971 to 1975 he had an intense relationship with french actress Catherine Deneuve. She was at his bedside when he died, along with their daughter, Chiara Mastroianni.

Visited the genius Sergei Parajanov at his house in Tbilisi, Georgia, Soviet Union in 1980s and then, in July of 1990, along with Federico Fellini, Tonino Guerra and Bernardo Bertolucci wrote: "With the death of Sergei Paradjanov cinema lost one of its magicians".

Federico Fellini nicknamed him "Snaporaz" while they are working on Dolce vita, La (1960); 20 years later this was the name of the character he played in Città delle donne, La (1980).

He is buried in the Cimitero Monumentale del Verano in Rome, Italy.