Owusu-Mainu was elected in the 2004 parliamentary election held on 7 December 2004, on the ticket of the National Democratic Congress, as the Member of Parliament for the Atebubu-Amantin Constituency after it was newly created in 2003. He thus became part of the Members of Parliament elected for the Fourth Parliament of the Fourth Republic of Ghana. He won with 53.5% of total votes cast against Mumuni Ibrahim Mohammed of the New Patriotic Party, who got 41.3% of total votes cast and Amankwah Kokro - an independent candidate, gaining 5.2% of total votes cast. The National Democratic Congress won 10 parliamentary seats out of 24 parliamentary seats for the Brong Ahafo region in that elections. In total, the party won a minority representation in the 4th parliament of the 4th republic with 94 seats out of 230 total seats in that election of 2004.
