In real-life, Anthony Quinn was a close personal friend of gangster Frank Costello after the mob boss retired and until Costello's death.
Even though the others constantly call Benjamin Siegel "Bugsy", in real-life, Siegel never liked the nickname and would usually get upset and even violent if someone used the name in his presence.
At the banquet where Don Faranzano announces himself as the "Boss of all bosses", he names the underbosses as "Lucky" Luciano, Joe Bonnano, Joe Profaci, Vincent Mangano, and Thomas Gagliano. After Lucky orchestrated the death of Faranzano, the five underbosses became the heads of the individual "Five Families" of organized crime in New York City. Eventually, through years of succession, the Luciano family became the Genovese Family, the Mangano family became the Gambino Family, the Gagliano family became the Luchese Family, and the Profaci family became the Columbo Family. Only the Bonnano Family retained the name of its original leader.
Mara was loosely based on Gay Orlova, a chorus girl and escort Charlie "Lucky" Luciano dated while in New York City.