In 1984 Montag joined the Green Party. In the years 1998 to 2002 he was chairman of the party in the State of Bavaria. Since 2002 Montag has been a member of the Bundestag, elected from the Bavarian regional list, after unsuccessfully contesting the Munich South constituency, and speaker of the Green Party's parliamentary group on law politics. In 2005 he was spokesman of the Greens during the meetings of the Committee of inquiry on the German Visa Affair 2005.