She was originally elected in 2001, to represent the Peter-McGill district as a councillor under Gérald Tremblay's Union Montreal party.
Tremblay named her to the city's executive committee, where she was responsible for the social development/revenue security and the Business Centre portfolios.
[1] Due to a disagreement with the Tremblay administration, she resigned from the executive committee and Union Montréal and spent the rest of her time in council sitting as an independent.
[1] During the 2006 Canadian federal election, O'Sullivan ran for the Conservative Party of Canada in the riding of Westmount—Ville-Marie.
Louise O'Sullivan ran as a mayoral candidate in Montreal during the 2009 municipal elections for Parti Montréal Ville-Marie.