Born in Genoa but raised in Rome, Gassman first studied for a law degree before attending the National Dramatic Academy. He joined the Elsa Mellini Company in 1943 and quickly became established as a rising star, appearing in numerous classical roles. His striking good looks and talent were soon put to use in Italian cinema playing the romantic figure in a number of period pieces. His performance as Silvana Mangano's lover in his fourth film, "Riso amaro/Bitter Rice" (1949), made him an international star. The pair proved so popular they were reunited for 1951's "Anna", in which Gassman undertook a less heroic role.

Inevitably, Hollywood beckoned and Gassman was put under contract at MGM but the studio did not know how best to exploit the actor's capabilities, relegating him to roles as stereotypical Europeans in such fare as "The Glass Wall" (1953) and "Mambo" (1954), the latter opposite his then-wife Shelley Winters. His last US film for many years was a turn as Anatole in King Vidor's filming of "War of Peace" (1956).

Returning to Europe, Gassman undertook a dream project, writing, starring and co-directing (with Francesco Rosi) "Kean" (1956), a biopic of the legendary British actor Edmund Kean. Many critics noted Gassman's tendency to overact in the role and that hammy quality has informed many of his subsequent film roles. But he could rein in his talent to offer fine comic turns as one of a group of bumbling thieves in "Big Deal on Madonna Street" (1958) and a jet-setting playboy in "The Easy Life" (1961). (The latter was one of 16 film projects he collaborated on with director Dino Risi.) Throughout much of the early 1960s, Gassman divided his time between his Teatro Popolare Italiano and the cinema. On stage, he tackled roles from Agamemnon in "Oresteia" to several in the Shakespearean canon while on screen he was usually seen in comic roles. A career highlight came with his turn as a blind army captain in 1975's "Profumo di donne/Scent of a Woman" for which he was named the year's best actor at the Cannes Film Festival. (The film was the basis for the 1992 Americanized remake that starred Al Pacino.) After neatly 30 years, the actor returned to US films as the groom's father in Robert Altman's uneven "A Wedding" (1978) and was wasted in Altman's dreadful "Quintet" (1979). A number of his other American features were equally unworthy (e.g., "The Nude Bomb" 1980; "Tempest" 1983). In 1983, he and his son Alessandro co-wrote and co-starred in "Di Padre in figlio/From the Father to the Son". Four years later, he shone in dual roles as a patriarch and the patriarch's father in flashbacks in Ettore Scola's "La Famiglia/The Family" (1987). As the 90s dawned, Gassman's film work dwindled a bit, but he offered a fine turn as an aging crime lord in Barry Levinson's "Sleepers" (1996).