In the leadership election of 1985, Johnson was chosen to succeed PQ founder René Lévesque as leader of the party and consequently as Premier of Quebec.

[4] In 2001 he was appointed as chief advisor and negotiator of the Quebec government in the Softwood Lumber dispute between Canada and the United States by then Premier Bernard Landry.

In October 2006, he was chosen by the Charest government to preside over a public inquiry into the collapse of a viaduct over Autoroute 19 in Laval, Quebec, leaving five dead and six injured.

The choice of Johnson was criticized by both leaders in opposition André Boisclair (PQ) and Mario Dumont (Action démocratique du Québec) because of the possibility of conflict of interest.

[5] Johnson was appointed by the minority Conservative government to the Canadian delegation at the United Nations' Bali Conference on climate change.

In December 2005 he made waves in sovereigntist circles by supporting Liberal candidate and close, longtime friend Raymond Bachand in a provincial by-election in the Outremont riding.