The Castellammarese War continues as mob boss Joe Masseria offers to negotiate with his rival, Salvatore Maranzano. Maranzano and the Castellammarese faction refuse to end the war, demanding the death of Masseria ally Joe "The Baker" Catania. Although Catania would be murdered later in 1931, the war would continue. Mobster Salvatore Sabella is arrested for assault and battery using a motor vehicle; however, he receives a suspended sentence. During his indictment, New Jersey police discover that his address matches that of Stefano Magaddino, leader of the Buffalo crime family, following Magaddino's arrest in 1921. Shortly after his arrest, the 40 year old Sabella retires as leader of the Philadelphia crime syndicate in favor of lieutenant John Avena. Charles Entratta, a partner of New York gangster Jack "Legs" Diamond is gunned down in a Brooklyn neighborhood.
February 3 - Joe Catania is killed as he leaves his home in New York. It is thought that Masseria ordered Catena's murder out of suspicion that he was hijacking Masseria's liquor shipments.
April 15 - Mob boss Joe Masseria is killed by Joe Adonis, Vito Genovese, Albert Anastasia, and Bugsy Siegel on the orders of Lucky Luciano. Ciro Terranova drives the getaway car. This effectively ends the long Castellammarese War in New York. May - Following Masseria's death, Salvatore Maranzano declares himself Capo di Tutti Capi (boss of bosses) during a mob conference in Chicago, Illinois. While the conference was to serve as a reconciliation with Al Capone's Chicago Outfit, ally Charles Luciano and others conspire to eliminate Maranzano.
September 10 - On orders of Charles Luciano and Frank Costello, boss of all bosses Salvatore Maranzano is murdered in his headquarters on Park Avenue in Manhattan by gangsters disguised as police officers. That same day, several of Maranzano's lieutenants, including James Marino, are killed by unknown gunmen including outside a Bronx neighborhood barbershop. The bodies of Maranzano allies Samuel Monaco and Louis Russo would later be recovered from Newark Bay; both corpses would show signs of torture. These events may or may not have been the basis for the beginning of the alleged "Night of the Sicilian Vespers" in which many old world Sicilian-born mafiosi are killed throughout the country by the Luciano-Lansky faction in the aftermath of the Castellammarese War.
September 17 - Meyer Shapiro, who controls bootlegging, illegal gambling and prostitution in New York's East Side, is murdered by former associates Abe "Kid Twist" Reles and Martin "Bugsy" Goldstein, who quickly take control of his criminal operations. His younger brother Irving was previously killed outside his Bronx apartment on July 11. One year later, Willie Shapiro would also be killed, buried alive by Reles and his associates.
October 6-24 - Chicago Outfit boss Al Capone is convicted of tax evasion, fined $50,000, and sentenced to eleven years in a federal prison. October 15 - Joe Ardizonne, head of the Los Angeles crime family, disappears and is presumed murdered (possibly a victim of the "Night of the Sicilian Vespers"). Mobster Jack Dragna would go on to succeed Ardizzone as leader of the Los Angeles family.
December 18 - New York Prohibition gangster Jack "Legs" Diamond is shot to death while staying at a safe house in Albany, New York by a number of unidentified gunman. Diamond is one of the last major Irish-American gangsters in New York to be eliminated by the Italian-American "National Crime Syndicate".
December 22 - Irish-American mob boss Frankie Wallace, on the pretense of a sit-down with Italian-American mobsters, is ambushed and murdered in Boston's North End. The Gustin Gang would be the first victims in a three-year systematic elimination of Irish-American gangsters in the United States.