Mr Johnson, a 44-year-old lawyer and businessman, was the only candidate in the process to choose a new leader for the Quebec Liberal Party, who automatically succeeds Mr Bourassa as provincial premier. Mr Bourassa, who has been battling against skin cancer, decided his health would not permit him to fight the next election which must be called before next August.
Although the Johnson name is prominent in Quebec politics - his father was premier of the province in the 1960s and his brother was briefly the leader of the Parti Quebecois and premier following the death of its founder, Rene Levesque - the new Liberal leader is seen more as a manager than as a politician.
Mr Johnson inherits a dispirited and beleaguered government. It has been trailing the separatist-oriented Parti Quebecois by a wide margin in the polls and many of Mr Bourassa's senior ministers have announced they will step down with him.
The situation facing the Quebec Liberals is parallel to that facing the federal Conservative Party when the former prime minister Brian Mulroney quit earlier this year, and two-thirds of his ministers chose not to run in the recent election. But the Quebec political landscape is even more cluttered than the federal scene. As Mr Johnson's leadership became official on Wednesday, a breakaway group of Liberals announced the formation of Parti Action Quebec.
Led by Jean Allaire, who had been Mr Bourassa's constitutional advisor until he quit the Liberals in protest at the premier's approach to last year's constitutional negotiations, the new party will seek support for a compromise between the Parti Quebecois' hardline separatism and the Liberals' federalism under Mr Bourassa, an approach Mr Johnson has promised to make explicit.
Mr Allaire's new group is promoting Quebec sovereignty within a looser economic federation similar to the European Union. The new party has both Liberal and Parti Quebecois supporters worried, since opinion polls suggest there is up to 30 per cent support for the middle option and no one is certain which party will be hurt most.
The Parti Quebecois is counting on a two-step process to lead Quebec out of the federation. The first would be winning the coming election, then a referendum.
Adding to the uncertainty is what role the Bloc Quebecois, now the official opposition in the national parliament, will play. The Bloc leader, Lucien Bouchard, has promised to defend Quebec's interests within the national government. Mr Bouchard, a more charismatic presence than Jacques Parizeau, the Parti Quebecois leader, is eyeing a role for Quebec, as differentiated from Ottawa, in the longer term.
Mr Johnson's best hope for hanging on to power is Mr Parizeau's personal unpopularity, while the separatists might have to wait until they can replace Mr Parizeau with Mr Bouchard to further their goal.