Johnston was born in Kirkintilloch, Scotland. After attending Glasgow University, he established the left-wing newspaper, Forward, in 1906. It was his editorship of Forward that brought him to prominence in the Scottish left.
A member of the Independent Labour Party, Johnston was elected to the House of Commons at the 1922 General Election as part of the Labour group. Also successful were several of his socialist colleagues that he had been working with previously in Glasgow, such as John Wheatley, Emanuel Shinwell and James Maxton.
Johnston became a strong opponent of MacDonald's National Government, refusing to serve as part of it along with the vast majority of his Parliamentary Labour Party colleagues. He lost his seat in the 1931 General Election.
Johnston returned to the House of Commons in 1935. During the Second World War he was appointed Secretary of State for Scotland by Winston Churchill.
After ceasing to be a Member of Parliament he held various posts with different public bodies. Tom Johnston died on 5 September, 1965.