Later on he became passionate about boxing and started competing in illegal prize fights in Los Angeles. At 15 he left for New York to begin a career as a professional boxer. He was soon matched up against fighters such as Patsy Farr, Chalky Wright and Baby Arizmendi. With a less than stellar record of 76 wins, 29 losses and 16 draws, Cohen hung up his gloves in 1933.
Cohen moved to Chicago and became an enforcer in Al Capone's gang. He also began running card games and engaging in other illegal activities. While there he survived arrests, prison and at least one assassination attempt. After this attempt on his life he moved back Los Angeles.
In Los Angeles, Cohen partnered with Bugsy Siegel acting as the extension of the East Coast syndicate. There they controlled narcotics, gambling and union operations. Cohen also helped Siegel create and then run The Flamingo Hotel.
After Siegel's murder in 1947, Cohen became the top West Coast mobster although not to the liking of rival Jack Dragna who had made his way back to Los Angeles after Siegel's death. During the wars between Cohen and Dragna, he survived numerous attempts on his life and even a bombing of his Brentwood house.
In the early 1950's after being investigated by the Kefauver Commission, Cohen was sentenced to four years in prison for tax evasion. After his release he became a media celebrity, even appearing on the Mike Wallace show in the late 1950s. In 1961 he was again convicted of tax evasion and sentenced to 15 years. First sent to Alcatraz, he was soon attacked with a lead pipe by another inmate, causing Cohen's partial paralysis.
After his final release from prison in 1972, Cohen came to be thought of as an elder statesman and "gentleman mobster." He associated with luminaries such as Richard Nixon, Sammy Davis Jr, Billy Graham and Randolph Hearst, even appearing on the Merv Griffin Show, although, no doubt, still engaging in nefarious activities.
Cohen died in 1976 of natural causes.