Fair question. Although the Outfit wasn't Mafia under Capone, Charlie Luciano prudently invited Capone to sit on the Commission--he even named him chairman. It was an "honorary" title. Therefore, you might make the assumption that the Outfit, as a Commission member, was considered "Mafia" as far back as 1931.
Capone kept trying to install his own guys as heads of the Unione Siciliana, which was technically the real Mafia in Chicago. He succeeded periodically. I'm guessing that the Unione was absorbed into the Outfit, and Chicago considered itself Mafia. But Chicago continued the tradition of having non-Italians in high places.