 Gérald Tremblay's Montreal Island Citizens Union (MICU) won a majority in this election, and Thériault served as a member of the opposition.By virtue of her position on city council, she also served on the Mercier–Hochelaga-Maisonneuve borough council.Thériault was elected as borough mayor of Mercier–Hochelaga-Maisonneuve in the 2005 municipal election, defeating Pierre Bélanger, a former Parti Québécois (PQ) cabinet minister who ran for MICU.