Sonny Forelli is the Don of the Forelli Family of the Liberty City Mafia circa 1986 - the richest and most powerful Family in Liberty City in the 80s. Sonny came to power young, and ordered several important mob killings around the time of Tommy Vercetti's imprisonment that resulted in the great success of the Forelli Family. At the time of Grand Theft Auto: Vice City, Sonny is believed to have major influences in racketeering, gambling, unions, corruption and prostitution, with growing interest in the narcotics trade (which the other Mafia Families have tried to avoid).
Tommy Vercetti grew up with Sonny and they were friends. Sonny granted Tommy a position in the organization and he was loyal to the Family above all else. Then, fifteen years before the start of GTA Vice City, Tommy was sent by Sonny to the Harwood District to assassinate a key mobster. Tommy was apparently ambushed, and ended up killing eleven rival mobsters in a massacre earning him the title of the "Harwood Butcher". Tommy always believed that Sonny set him up, although his motives were unknown.
By the time the game starts, Sonny Forelli is wary of the possibility of other gangs, including the rival Families, cutting into Vice City's hugely profitable and vastly expanding drug business, and wants to stake a claim for the Forelli Family first. At the same time, Tommy Vercetti is back in town, just fresh from prison. Because Tommy kept quiet when he did his time, Sonny feels obliged to give him a job; however, Sonny believes Tommy is still too well-known as the "Harwood Butcher" and he feels this will prove harmful to business in Liberty City. He then sends Tommy to Vice City as a formal Forelli Family representative to set up a drug outpost there. His intention was for Vercetti to start a large drug cartel in Vice City operating on behalf of the Forelli Family.
When it is learned that Vercetti emerges without the millions in cash or the drugs he was sent to buy as a result of the ambush during a drug deal, Sonny is enraged and promises to kill Tommy if he doesn't get both the money and the merchandise back.
As Tommy Vercetti grows powerful and builds his extensive empire, Sonny calls in periodically to pressure Tommy into giving him his cut. Once it is clear that Tommy has gone solo, he sends enforcers to Vice City to rough up Tommy's men and tax his multiple businesses. When Tommy slaughters all of them, Forelli approaches (or was approached by) one of Tommy's partners, Lance Vance, and the two conspire against Tommy.
Tommy was expecting Sonny and his men when they arrived at his mansion's doors, ready with suitcases full of counterfeit money to pay the Forelli Family off once and for all. When Lance Vance tells Sonny the real money is in Tommy's office safe and Sonny reveals that Tommy's imprisonment was actually organized by Sonny, Tommy realizes the time for diplomacy is over. A huge gunfight ensues, and Tommy slaughters countless Forelli Family members, and the traitor Lance, within the mansion, ending with a showdown between Sonny and himself in the mansion's entrance hall. Tommy finally gets revenge for his 15-year imprisonment by personally killing Sonny.
The Introduction, Grand Theft Auto: San Andreas's prequel film, implied that the death of Sonny Forelli struck a severe blow to the Forelli Family and resulted in the incursion of the Sindacco Family into Liberty City from Las Venturas. Leone Don Salvatore Leone has referenced Sonny's name when responding in anger to the Underboss of the Sindacco Family, Johnny Sindacco.
Sonny Forelli was voiced by Tom Sizemore. Of minor note is that Sonny's name is also similar to that of Miami Vice character, Sonny Crockett. Sonny Forelli's violent temper is similar to that of Santino "Sonny" Corleone, a fictional character and son of Don Vito Corleone, in The Godfather novel and film. Sonny Corleone also speaks out against his father, voicing an opinion to become involved in the heroin trade.