Giannini studied at the Accademia Nazionale in Rome, and made his film debut in a small part in Fango sulla metropoli in 1965. He appeared in supporting roles in Anzio and The Secret of Santa Vittoria, and starred in the original version of Swept Away.

In 1976, he starred in Seven Beauties, for which he was nominated for the Academy Award for Best Actor, which is unusual in that his performance was given entirely in Italian. He dubbed Jack Nicholson's voice in the Italian release of The Shining, for which Nicholson sent him congratulations.

His fluency in English has brought him a number of featured roles in Hollywood productions, most notably as Inspector Pazzi in Hannibal. He has also appeared in A Walk in the Clouds and Man on Fire.

Giannini's best-known starring roles have been in films directed by Lina Wertmuller. In addition to Swept Away and Seven Beauties, he also appeared in The Seduction of Mimi, Love and Anarchy, A Night Full of Rain, and Francesca e Nunziata.

He played the padisha emperor Shaddam IV in the 2000 Dune miniseries. In 2002, he starred in the horror film Darkness. He was just signed on as Rene Mathis in Casino Royale.