Written by Dashiell Hammett (The Maltese Falcon) and drawn by Flash Gordon artist Alex Raymond, Secret Agent X-9 was introduced by King Features in January 1934.

X-9 is a loner who fights urban criminals by seeming to become part of their evil world. Hammett and Raymond each soon went on to other things in 1935, and the strip was continued by an assortment of writers and artists. It evolved into Secret Agent Corrigan in 1967 and was taken over by writer Archie Goodwin and artist Al Williamson, who would later collaborate on the Star Wars newspaper strip as well.

Comic book reprints of the first eight months of the strip were published in 1934. There was a short-lived radio program, and Universal Pictures made two Secret Agent X-9 chapter plays, one in 1937 starring Scott Kolk; the other, with Lloyd Bridges starring, in 1945.