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Upon his January 1921 election as Governor of Illinois, Lennington Small would begin issuing over 1,000 pardons to Chicagocriminals during his seven years in office, until his own indictment for fraud.
Sangerman's Bombers rise to prominence soon after the 1921 arrest and imprisonment of James Sweeney, leader of Sweeney's Bombers, a Chicagogang leader and professional bomber.
Chicago Black Hand leader, Sam Cardinelli, and fellow mobsters Nicholas "The Choir Boy" Viana and Frank Campione executed for the murder of saloon owner Andrew P. Bowman.
A major gang war breaks out in California between the Suey Sing, Bing Kong, Jung Ying and Suey Don tongs.
Carlo Gambino, the future founder of the Gambino crime family, arrives in New York as a stowaway from Palermo, Sicily at the age of 19.
May 20 – Labor racketeerCornelius Shea is accused of leading a bombing campaign during a stationary engineer's strike, in 1920. Charges are never filed due to lack of evidence.
July – Steve Wisniewski, a Chicagogunman who had recently hijacked a North Side Gang beer shipment, is last seen with Hymie Weiss and presumably taken outside Chicago and killed. Upon Weiss's return he explained "I took Stevie for a one way ride." This is the first time a gangland killing is used as the phrase "one way ride" is still commonly used today to refer to this method.
August 14 – Joseph Sinacola is gunned down in front of his two children during the long running feud between Jim Powers and Phillip D'Andrea. Sinacola had been released from hospital just two weeks earlier, following a July 6 attempt on his life.
