Novel and film biography
In both the novel and the film, Sonny is the eldest of Vito Corleone's four children. Unlike his quiet, level-headed father, Sonny is hot-tempered and prone to violence.
At age 16, Sonny commits a robbery. When Peter Clemenza, Vito's right-hand man and Sonny's godfather, informs Vito about it, Vito demands his son explain himself. Sonny replies that he had witnessed Vito murder the "Black Hand" gangster Don Fanucci years before, and he now wants to join the "family business". Vito sends him to Clemenza for training.
Sonny "makes his bones" at age 19. By his mid-20s, he is promoted to a caporegime in the Corleone family. By the end of World War II, he is his father's underboss and heir apparent, popular and feared as a merciless killer with a short temper. Sonny is not without a sensitive side, however; at age 11, he brings home a homeless boy, Tom Hagen, demanding he be allowed to live with the family. Vito informally adopts Hagen, who eventually rises through the ranks to become Vito's consigliere . As the eldest child, Sonny acts as protector to his younger siblings and has a soft-spot for his youngest brother, Michael. Although he can beat up and kill other men without hesitation, he cannot bring himself to harm women, children, or anyone unable to defend themselves.
Sonny has four children with his wife Sandra. He also has several mistresses, including his sister Connie's friend Lucy Mancini. In the novel, Sandra allows – and is grateful for – his infidelities because she is unable to take the size of his penis.
Sonny's life is upturned in 1945, when drug lord Virgil "The Turk" Sollozzo, backed by the Tattaglia family, approaches Vito with an offer to enter the narcotics trade. During the meeting, Sonny speaks out of turn, expressing an interest in the deal that Vito declines. Vito later castigates Sonny for revealing his thoughts to an outsider. Sollozzo later attempts to have Vito assassinated, believing Sonny, as his father's successor, will bring the Corleone family into the drug trade.
The failed assassination attempt leaves Vito near death, making Sonny acting boss of the Corleone family. Sonny orders Clemenza to execute Vito's traitorous bodyguard Paulie Gatto. Sollozzo mounts a second assassination attempt on Vito at the hospital, which Michael prevents. Sonny then orders Bruno Tattaglia, son and underboss of Sollozzo's ally Philip Tattaglia, to be murdered. Sollozzo proposes that Michael be sent to promote a truce. Sonny, believing it is a trick, refuses and demands that the other Mafia families hand over Sollozzo to the Corleone family or face war. Hagen successfully convinces Sonny to wait, because Captain Mark McCluskey, a corrupt NYPD captain on Sollozzo's payroll, has agreed to be Sollozzo's bodyguard. Hagen warns Sonny that killing McCluskey would violate a longstanding Mafia rule not to kill members of law enforcement, and that the backlash from rival Mafia families and law enforcement would be severe.
Michael, who had before wanted nothing to do with the family business, volunteers to kill Sollozzo and McCluskey, arguing that McCluskey is fair game because he is a corrupt cop mixed up in the drug trade. Sonny is impressed with Michael's confidence and seriousness, but doubts that his "nice college boy" brother is capable of murder. Ultimately, however, he approves the hit. Michael meets with Sollozzo and McCluskey at an Italian restaurant in the Bronx and kills both men. This ignites the New York underworld's first Mafia war in a decade. Sonny arranges for Michael to flee to Sicily under the protection of Vito's friend Don Tommasino.
The war between the Five Families drags on, and Sonny, unable to break the stalemate, orders bloody raids that earn him a legendary reputation. He also begins plotting to take out the rival Mafia bosses. Emilio Barzini, Vito's rival and the power behind Sollozzo, enlists Connie's abusive husband, Carlo Rizzi, to help set a trap. Earlier, Sonny assaulted Carlo after he attacked Connie. To draw Sonny out into the open, Carlo provokes Connie into an argument before severely beating her. Hysterical and in pain, she telephones the Corleone compound, asking Sonny for help. Enraged, Sonny speeds towards Connie's apartment in Hell's Kitchen ahead of his bodyguards. At the Long Beach Causeway toll plaza, Barzini's men trap Sonny and shoot him to death.
During a meeting with the other crime family dons to establish peace, Vito realizes that Barzini masterminded Sonny's murder. After Michael returns from Sicily, he assumes Sonny's place as Vito's heir apparent. Vito and Michael secretly continue Sonny's plot to wipe out the other New York dons to avenge Sonny's death and eliminate their rivals once and for all. After Vito dies in 1955 from a heart attack, Michael has the other dons murdered, including Barzini, who is shot twice in the back on the steps of a courthouse in broad daylight. Michael informs Carlo that the rival dons have been killed and coerces Carlo's confession for his part in arranging Sonny's murder. Carlo admits Barzini was the mastermind. On Michael's orders, Clemenza garrotes Carlo to death. With that strike, the Corleones become the most powerful crime family in the country.
