Mawhinney married Maureen Gribben (born 5 January 1943) on 4 April 1967. 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.
In 1987, Mawhinney was elected as Deputy Leader of the Alliance Party, a position which he held until he resigned in 1991, claiming "health and business reasons". In 1989, he finally won a seat on Newtownabbey Borough Council, in Manse Road In 1993, he instead won a University seat, but he did not restand in 1997.
- Who's who in European Politics (1982, p406)
- The Local Government Elections 1973-1981: Newtownabbey, Northern Ireland Elections
- South Antrim 1973-82, Northern Ireland Elections
- South Antrim 1983-1992, Northern Ireland Elections
- Sydney Elliott and William D. Flackes, Northern Ireland: A Political Directory, 1968-1999
- Local Government Elections 1985 - 1989: Newtownabbey, Northern Ireland Elections
- Newtownabbey Borough Council Elections 1993-2005, Northern Ireland Elections
|Northern Ireland Assembly (1982)|
|MPA for South Antrim
1982 - 1986
|Party political offices|
|Deputy Leader of the Alliance Party of Northern Ireland
1987 - 91