Marijuana was not illegal in the United States until October 1, 1937, when Congress passed the "Marijuana Tax Act".  Total debate time on the House of Representatives floor concerning this issue: 90 seconds.  This act did not actually ban the substance - it simply said that one could not sell marijuana without a license.  Of course, Congress refused to issue any licenses.  Congress finally banned marijuana outright in 1970. 

Source: The Unbelievable Truth! by Jeff Rovin, 1994, Signet Books