Born in 1916, Sonny is the most impulsive and violent of Vito's children and, before Michael's rise to power, the most involved in his father's criminal operations. As the oldest child, he serves as a protector to his younger siblings. When Sonny was 16, he committed a robbery. His father learns about it from Peter Clemenza, who stood as godfather to Sonny. When Vito asks Sonny why he did this, Sonny tells him that he saw his father kill Don Fanucci back in 1920. He then tells his father, "I want to learn how to sell olive oil." Vito realizes what Sonny really means, and sends him to Clemenza for training. Sonny "made his bones" when he was 19.
Sonny eventually becomes a capo in his father's family, gaining a reputation as a ruthless killer. By the end of World War II, he has become his father's underboss and heir apparent. He is not without a softer side, however; at the age of 11, he meets a homeless boy, Tom Hagen, who thereafter lives with the family and becomes the family's consigliere. He also acts as a protector to his younger siblings, especially Connie. The book reveals that he secretly cannot bring himself to harm women or children, or anyone who can't defend themselves, which may be what saves his sister's husband, Carlo Rizzi.
Although Sonny has a wife, Sandra, and four children, he frequently cheats on Sandra with other women. At the time of the film, he has carried on a long-running affair with Lucy Mancini, who served as one of his sister's bridesmaids. The normal course of events in Sonnys life is upturned when Virgil 'The Turk' Sollozzo comes to Don Vito with an offer of entering the drug business, backed by the Tattaglia family. Vito rejects the deal, although Sonny shows slight interest. Sollozzo responds by trying to assassinate Vito, in hopes that Sonny will take over the Family and go into the drug business.
The assassination attempt fails but leaves Vito near death, although he eventually recovers. Sonny takes over as acting boss and issues an ultimatum to the Tattaglias--turn over Sollozzo or face war. When he learns that Sollozzo instead had his father's top button man, Luca Brasi, killed, he orders Bruno Tattaglia, Don Philip Tattaglia's son, killed in response.
Michael, who had previously distanced himself from the family's criminal enterprise, volunteers to kill Sollozzo and his ally, police Captain McCluskey. It is a hard and fast rule in the Mafia that policemen are not to be harmed, but Michael argues that McCluskey is fair game since he is serving as Sollozzo's bodyguard. The normally violent Sonny is initially amused by this, believing Michael is taking McCluskey breaking his jaw the night before too personally. However, convinces him that it's in the family's best interest to kill Sollozzo and McCluskey ("It's not personal, it's strictly business."). Michael kills Sollozzo and McCluskey at Louis's Restaurant in the Bronx and is sent immediately to Sicily to wait out the inevitable crackdown on the Five Families.
In retaliation, during the spring of 1948, Tattaglia's partner, Don Emilio Barzini, enlists the help of Sonny's brother-in-law, Carlo Rizzi, in setting a trap for Sonny. Sonny had already given Carlo a savage beating upon learning that Carlo was abusing Connie, unintentionally revealing a weakness. To draw Sonny out into the open, a vengeful Rizzi inflicts a particularly vicious beating on Connie, who telephones Sonny, begging for help. In a fit of rage, Sonny speeds out of the family compound unaccompanied, and heads for Connie's apartment in Hell's Kitchen to confront Rizzi. As Sonny approaches a toll plaza, a number of Barzini's men emerge with Tommy Guns and brutally murder him. As a result, Don Corleone promises not to seek vengeance for his son's death in exchange for stepping down and Michael's safe return home. Later, Michael, who was not part of his father's promise, has all of his opponents assassinated and has Carlo garroted to death in revenge.
Role in Godfather sequels
Sonny appears in the original Godfather and in its sequel, The Godfather Part II. In the latter film, he briefly appears in a flashback scene portraying the family dinner in which Michael announces he is volunteering to fight in World War II. He is angered by Michael's decision, and he berates his brother for risking his life "for a bunch of strangers." The flashback reveals that Sonny introduced Carlo to Connie and the rest of the family, hence their later marriage.
Vincent Mancini, who eventually succeeded Michael as head of the Corleone family, is the illegitimate son of Sonny and Lucy Mancini in The Godfather Part III. Vincent does not exist in the books, as it was specifically mentioned in Puzo's original novel that Lucy never bore a son with Sonny.