Johnston had become a supporter of the
Labour Party and on 29 October 1947 he was appointed as
Solicitor General for Scotland, a government position. He was not then a Member of Parliament but when the constituency of
Paisley was vacated by
Viscount Corvedale on inheriting his father's Earldom, Johnston was selected to follow him as Labour candidate on 8 January 1948. In the
ensuing by-election he faced a straight fight with J.M. MacCormick, a Glasgow solicitor who had been a prominent Scottish Nationalist and was adopted as candidate both by the Conservatives and Liberals. Johnston won by 6,545 votes. Johnston retained his government post until Labour went out of office in 1951. He then became an opposition front-bencher. In May 1954 he moved the rejection of the Town and Country Planning (Scotland) Bill on the grounds that it burdened the community with the cost of compensating landowners for the loss of land value. He was not a prominent figure, concentrating on the details of policy. In the meantime he kept up his practice at the Scottish Bar, often appearing at public inquiries. ==Judicial post==