He was selected to fill the vacancy caused by the murder of Robert Bradford. Smyth was consequently elected Member of Parliament in a 1982 by-election, receiving 17,123 votes. Later the same year, he was elected to the Northern Ireland Assembly, again polling double the electoral quota. He along with all other Unionist MPs resigned his seat in 1985 in protest at the Anglo-Irish Agreement. He successfully defended his seat in the subsequent by election. In his paper "A Federated People" (published by the Joint Unionist Working Party in 1987), Smyth proposed a federal United Kingdom with the state governments of England, Scotland, Wales and Northern Ireland each being autonomous from each other and, most significantly, fully independent from the federal parliament and government of the United Kingdom at Westminster.