Differences from the novel

One of the primary parts of Puzo's novel which was not used for the movie was the flashback story of Don Corleone's earlier life, including the circumstances of his emigration to America, his early family life, his murder of Don Fanucci, and his rise in importance in the mafia, all of which were later used in The Godfather Part II.

Many subplots were trimmed in the transition from the printed page to the screen, including: singer Johnny Fontane's misfortunes with women and his problems with his voice; Sonny's impulsive dabbling in street crime as a teenager and his utter lack of the tact and coolheadedness possessed in such abundance by his father; Sonny's paramour Lucy Mancini's new-found love in Dr. Jules Segal (a character entirely missing from the film), who not only repairs Lucy's vaginal malformation but puts Michael in touch with the surgeon who repairs Michael's facial bones (which had been damaged by Capt. McCluskey) and also operated on Johnny Fontane's vocal cords, thus restoring his singing voice; Jack Woltz' increasing pedophilia; Kay Adams's home life; Luca Brasi's demonic past; the Corleone family's victorious rise to power in earlier New York gang wars in which Don Corleone survives a previous assassination attempt and Al Capone sends triggermen from Chicago in an unsuccessful attempt to aid a rival gang; Don Corleone's ingenious plan used to take Michael out of exile in Sicily; the detailed savage attack on the two men who assaulted Bonasera's daughter, which was led by Paulie Gatto and involved retainer thugs (which was only alluded to in the film).

Additionally, the novel makes it clear that Lucy was not pregnant by Sonny when she moved to Las Vegas, thus leaving no room for Vincent Mancini of The Godfather, Part III. Curiously, Puzo wrote the screenplays of all three movies, so the contradiction was well known to him. Characters with smaller roles in the film than in the novel include Johnny Fontane, Lucy Mancini, Rocco Lampone, and Al Neri (the latter two are reduced to non-speaking roles). Characters dropped in the film adaptation beside Dr. Segal include Genco Abbandando (only spoken of, he appears in a deleted scene featured in The Godfather Saga; he first appears on film in The Godfather II) and Dr. Taza from Sicily. Also, in the book, Michael and Kay have two sons, but in the movies they have a son and a daughter.

The novel and film also differ on the fates of Michael's bodyguards in Sicily, Fabrizzio and Calo. The film has them both surviving (Calo, in fact, appears in the third installment). In the book, however, Calo dies along with Apollonia in the car explosion, and Fabrizzio is shot and killed as one more victim in the famous "baptism scene" after he is tracked down running a pizza parlor in America. Fabrizzio's murder was deleted from the film but publicity photos of the scene exist.(He is later killed in a completely different scene in The Godfather Saga which was deleted from The Godfather: Part II).

The ending of the book differs from the end of the movie: whereas in the film Kay suddenly realizes that Michael has become "like his family", the drama is toned down in the book, where Tom Hagen lets her in on secrets for which, according to him, he would be killed should Michael find out. During the film's baptism scene, all the heads of the Five Families were killed. In the novel, only Barzini and Tattaglia, previously at war with the Corleones, are killed.