Greenbaum first came to prominence as an associate of Meyer Lansky. Greenbaum joined his organization on New York's Lower East Side in the mid to late 1910s. During Prohibition, Greenbaum began working with the Chicago Outfit managing the southwest division of the Trans-America Race wire service in 1928. Greenbaum was sent to Las Vegas shortly after World War II began. He gained control over syndicate gambling operations with Morris Rosen and Moe Sedway. Sedway and Greenbaum ran the El Cortez Casino in 1945, until Greenbaum was asked by William R. Wilkerson to manage casino operations at the Flamingo Hotel. In 1946, Bugsy Siegel took over construction and creative control of the Flamingo, until it was shut down in January, 1947, due to mounting losses as a result of Siegel's alleged skimmi