Billy Van Arsdale is born in New York in 1935. His family makes a fortune selling citrus juice for many businessmen, including Vito Corleone. He meets Vito Corleone's granddaughter, Francesca, in college in 1955; they are married a couple of years later after Francesca gives birth to their son, William Brewster Van Arsdale IV, whom they call 'Sonny', after Francesca's late father, Santino "Sonny" Corleone. He starts working as a Law Assistant in the Attorney General's office after graduating, due to his family connections. Shortly after, it is revealed that he has secretly been collecting evidence against the Corleone Family, most likely due to a grudge he has against Michael Corleone because he had his leg broken for infidelity.
By 1961, he begins an affair with one of Danny Shea's secretaries. Francesca never knows about this and thinks he would not have sex with her simply because she is pregnant. In early 1962, Francesca learns of her husband's infidelity and spying on her family. Distraught, she follows him in his own car one night, she finds him drunkenly stumbling out of a local bar, so she runs him down in a fit of rage, literally cutting him in half. She then leaves the scene and calls her Uncle Michael for help.
It is later revealed that Van Arsdale's mistress is eventually convicted of second-degree murder for his death; it is implied that she is framed by the Corleone family to keep Francesca safe.