After growing up in Dungannon, where he studied at the Christian Brothers' School, Stewart worked as a wine merchant and became active in the Irish Parliamentry Party. At the January 1910 UK general election, he worked as the election agent for Thomas Kettle.
In 1923, Stewart was elected to the Dungannon Board of Governors, a post he held until the Board was dissolved in 1948. He was also elected to Tyrone County Council for the Nationalist Party, was a long-time member of Dungannon Urban District Council, and was the President of the Tyrone Ancient Order of Hibernians.
At the Northern Ireland general election, 1929, Stewart was elected for East Tyrone, a seat he held until his death in 1964.
Following the death of Joseph Devlin, Stewart was elected to Westminster at the Fermanagh and Tyrone by-election, 1934, but he stood down at the following year's general election.
In 1958, Stewart became the Chairman of the Nationalist Party at Stormont, a position he retained until his death.