Sonny (Santino) is the most impulsive and violent of Vito's children and, before Michael's rise to power, the most involved in the Corleone crime family. Sonny 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. He also acts as a protector to his younger siblings, especially Connie.

The normal course of events in Sonny’s 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 makes an assassination attempt on 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, now acting as the Corleone Family's Don, prepares for an all-out war against the Tattaglias and Sollozzo. Michael, who had previously distanced himself from the family's criminal enterprise, volunteers to kill Sollozzo and his ally, police Captain McCluskey. Sonny is initially against the idea, but Michael talks him into it. Michael kills Sollozzo and McClusky, and is sent immediately to Sicily to wait out the inevitable crackdown on the Five Families. Bruno Tattaglia, Don Philip Tattaglia's son, is also killed in his nightclub.
In retaliation, Tattaglia's partner, Don Emilio Barzini, enlists the help of Sonny's brother-in-law, Carlo Rizzi, in setting a trap for the impulsive new Don. 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 leaves the family compound unaccompanied and heads for Connie's apartment to confront Rizzi. As Sonny approaches a toll plaza, a number of Barzini's men emerge from the toll booths and car ahead of Sonny's with Thompson submachine guns and viciously gun him down after spraying his car with lead. Badly wounded, Sonny manages to exit his car through the passenger door and is shot dozens of times before collapsing. One of his assassins then fires another line of bullets along Sonny's body.

Santino 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.
In the novel as well as the films, Sonny is portrayed as a womanizer, despite being married. His wife refuses sleep with him because his large penis makes sex painful for her. His most prominent dalliance in the saga is with Lucy Mancini, a friend of Connie's. The Godfather Part III reveals that he fathered a child out of wedlock with Lucy (this was invented for the film, as Lucy in the novel does not have any children by him.) That child, Vincent 'Vinnie' Mancini-Corleone, grows up to succeed Michael as Godfather.