Born in Stornoway in 1920, Stewart became convinced of the case for Scottish independence at the 1935 General Election and joined the SNP the following year. Left-leaning by political conviction, he subsequently joined the Labour Party but remained a member for only a short time before rejoining the SNP.
He saw active service during the [{Second World War]] with the British Army, but remained a SNP member throughout. He returned to the Western Isles where he served as a councillor for many years (twice serving as the council's provost) before election as a SNP MP in the 1970 General Election. He was the SNP's first ever MP returned at a general election, and the last declared in 1970.
From 1970 till the February 1974 General Election he was the SNP's sole Westminster representative. At that election though he was joined by six other SNP MPs, and at the October General Election of that year this number increased to eleven. Stewart became the SNP parliamentary group leader, with Billy Wolfe as the SNP leader overall.
Stewart continued to represent the Western Isles until 1987 when he retired from front-line politics. At the General Election of that year his seat was captured by the Labour Party from the SNP and has been held by thyem since.
Stewart was working on an autobiography when he died in 1992. It was edited an completed by his sister, and published in 1994 as A Scot in Westminster.
It was Stewart who famously described the SNP as a "radical party, with a revolutionary aim".