Born in Fontana Liri, a small village in the Apennines, Mastroianni grew up in Turin and Rome. During World War II, he was interned in a Nazi prison, but escaped and hid in Venice.
In 1945, he started working for a film company, and began taking acting lessons. His film debut was in I Miserabili (from Victor Hugo's Les Misérables) in 1947.
He soon became a major international romantic star, starring in Big Deal on Madonna Street, and especially when Federico Fellini cast him in La Dolce Vita, with Anita Ekberg, in 1960.
He followed that with another starring role in 8 1/2.
Mastroianni was married to the Italian actress Flora Carabella (1926-1999 and appeared in many films, including "Lunatics and Lovers" and "A Night Full of Rain") from 1950 until his death; they had one child, Barbara. He also had a daughter, Chiara Mastroianni, with his longtime mistress, the actress Catherine Deneuve; both were at his bedside when he died of pancreatic cancer.