The move to reign in Laval was made official at a cabinet meeting where a former head of the Quebec provincial police was named as the city's trustee.
Florent Gagne, who is not a police officer but a sociologist by training, has served as a high-ranking civil servant in the provincial government.
Marois said the decision was necessary, albeit lamentable.
"It's terrible and disheartening and sad," she said in Montreal.
"But at the same time, I think it was a wise decision by the current mayor of Laval to ask for help from the government, to which we responded positively," she said.
The measure was imposed after the latest in a string of corruption allegations levelled against the city administration in the suburb north of Montreal.
Laval's former mayor is accused of running city hall like a criminal racket and he now faces gangsterism charges, after having resigned in scandal.
Now, the new interim mayor is alleged to have also taken part in illegal party financing. Alexandre Duplessis has denied that allegation, levelled last week in testimony at the Charbonneau inquiry.
However, Duplessis requested trusteeship for the city after the provincial government had already publicly raised it as a possible solution.
Municipal Affairs Minister Sylvain Gaudreault told a news conference in Quebec City that the trusteeship was in the best interests of the city.
"The decision follows a series of exceptional events that have marked the administration of the City of Laval and also seriously weakened the confidence of the citizens of Laval with regard to their local government," he said Monday.
Gaudreault said there is no time limit on the trusteeship but noted the municipal elections slated for November are a consideration.
"Certainly, the election in November will be an important event," he said.
Gagne and fellow trustees Sandra Bilodeau and Sylvie Pierard will be responsible for approving all the decisions and budgets of city council. They also have power over hiring and firing of staff.
At the moment, three municipalities are under trusteeship in the province but none is nearly as big as Laval.
— With files by Martin Ouellet in Quebec City
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