Johnson began working closely with other socialists in Glasgow including John Wheatley, Emanuel Shinwell, James Maxton, William Gallacher, John Muir, David Kirkwood, Jimmie Stewart, Neil Maclean, George Hardie, George Buchanan and James Welsh.

A member of the Labour Party, Johnston was elected to the House of Commons in November 1922. Also successful were several other militant socialists based in Glasgow including John Wheatley, Emanuel Shinwell, James Maxton, John Muir, David Kirkwood, Jimmie Stewart, Neil Maclean, George Hardie, George Buchanan and James Welsh.

When Ramsay MacDonald became prime minister following the 1929 General Election, he appointed Johnston as Under-Secretary of State for Scotland.

The election of the Labour Government in 1929 coincided with an economic depression and Ramsay MacDonald was faced with the problem of growing unemployment. MacDonald asked Sir George May, to form a committee to look into Britain's economic problem. When the May Committee produced its report in July, 1931, it suggested that the government should reduce its expenditure by £97,000,000, including a £67,000,000 cut in unemployment benefits. MacDonald, and his Chancellor of the Exchequer, Philip Snowden, accepted the report but when the matter was discussed by the Cabinet, the majority voted against the measures suggested by Sir George May.

Ramsay MacDonald was angry that his Cabinet had voted against him and decided to resign. When he saw George V that night, he was persuaded to head a new coalition government that would include Conservative and Liberal leaders as well as Labour ministers. Most of the Labour Cabinet totally rejected the idea and only three, Philip Snowden, Jimmy Thomas and John Sankey agreed to join the new government.

Johnston, a strong opponent of MacDonald's new government, lost his seat at Stirling and Clackmannan in the 1931 General Election.

Johnston returned to the House of Commons in November 1935. He held the seat until he retired in June 1945. After leaving Parliament he was Chairman of the Scottish National Forestry Commission (1945-48), Chairman of North of Scotland Hydro-Electric Board (1946-59), Chancellor of Aberdeen University (1951-65) and governor of the British Broadcasting Board (1955-56). Tom Johnston died on 5th September, 1965.