 In 1981, Mawhinney stood for the Alliance Party of Northern Ireland in Newtownabbey District "C", but was not elected.However, he was elected to the Northern Ireland Assembly, 1982, in South Antrim.He stood in the equivalent Westminster constituency at the 1983 general election, receiving 11.9% of the votes cast, and increased his share to 16.0% at the 1987 general election, in which he took second place.