He was elected to the National Assembly as a Liberal in 1981. Re-elected in the 1985 election that returned Robert BOURASSA to the premiership, Johnson was appointed minister of industry and commerce and deputy House leader. In 1988 he became president of the Treasury Board and was re-elected in 1989.
Following the resignation of Bourassa in 1993, Johnson was acclaimed as leader of the Québec Liberal Party on 14 December and was sworn in as premier on 11 January 1994. Bourassa's leadership had left the party trailing the Parti Québécois in the polls with an election due before the end of the year. Johnson performed well as premier but lost the September 12th election to Jacques PARIZEAU's PQ. Johnson won his seat and remained as Opposition leader, and became the leader of the federalist forces which eked out a narrow victory in the referendum on independence held 30 October 1995. After Lucien BOUCHARD took over the Parti Québécois, Johnsons Liberal Party languished and promised little hope of regaining power. Johnson resigned 2 March 1998 and was succeeded by Jean CHAREST.