Smyth stood for the Democratic Unionist Party (DUP) in North Antrim in the Northern Ireland Assembly election, 1973, narrowly missing out on a seat. Following the death of David McCarthy, an Ulster Unionist Party (UUP) MPA, he easily won a by-election for a seat on the Assembly. He also became the secretary of the United Ulster Unionist Council.
Smyth was elected to the Northern Ireland Constitutional Convention, again in North Antrim, in 1975. He was expelled from the DUP, however, in 1976, after he was accused of passing information to the then Secretary of State Merlyn Rees' office. He joined the Ulster Unionist Party (UUP) and stood unsuccessfully as its candidate for the Westminster seat of North Down at the 1979 general election.
From the 1980s onwards, Smyth turned increasingly to history and the Orange Order. He wrote a critical biography of Ian Paisley, Ian Paisley: Voice of Protestant Ulster. He was also a leading member of the Campaign for Equal Citizenship, which was founded in 1986, and campaigned for the three main British political parties to fully organise in Northern Ireland.
Smyth was also a history teacher at Knockbreda High School in Belfast for many years, and is also regarded as an expert on the Orange Order and its lodges.During the 1990s, Smyth resigned from the Orange Order. In 2005, he revealed that he had transvestite compulsions.
In 2011 it was revealed the BBC decided to drop him from its yearly live coverage of the twelfth of July parades in Belfast.Smyth who commentated on the programme for over 10 years was quoted as being 'sad' after being informed.
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