Johnston was born to a wealthy family in Bogarttown, Ontario that owned lumber and flour mills in Simcoe County.
[1] During the parliament, Johnston broke with both the government and the Liberals to support the new agrarian based Progressive Party formed by Thomas Crerar.
Prime Minister William Lyon Mackenzie King convinced Johnston to cross the floor and join the Liberals in early 1926 in an attempt to buttress his government's support.
[2] He was re-elected as a Liberal in the 1926 federal election and, in December 1926, became Deputy Speaker of the House of Commons of Canada.
[1] Johnston was appointed to the Senate of Canada by Mackenzie King in 1943 and sat in the upper house until his death in 1948.