Quebec Liberal Party

Dumont bought his first membership card in the Liberal Party of Quebec at age 15. He stated to his school friends that in the future he would be Quebec's premier.^[2] Dumont was a former President of the Liberal Party's Youth Commission, but had a falling out with the party following the rejection of the Allaire Report proposing maximalist powers for Quebec after the collapse of the Meech Lake Accord.

Dumont called himself and Michel Bissonnet, who preceded Dumont as leader of the Liberal youth wing, the first Liberal sovereigntists. This was the group that led the Liberal sovereigntist faction while Liberal premier Robert Bourassa remained unopposed.^[3]

Dumont organized the "Liberals for the No side," in the 1992 referendum on the Charlottetown Accord.

Action démocratique du Québec

Dumont and Liberal party insider Jean Allaire played a central role in the creation and development of the Action démocratique du Québec (ADQ) in 1994. He succeeded Allaire as leader after the latter resigned for health reasons.

Dumont was elected as an ADQ member of the National Assembly for Rivière du Loup in the 1994, 1998, 2003, 2007 and 2008 elections.

In the 1995 Quebec referendum, Dumont joined with Parti Québécois Premier Jacques Parizeau and Bloc Québécois leader Lucien Bouchard, who wooed him and with whom he remains close, in support of the sovereigntist campaign.^[4] Dumont revealed to Journalist Chantel Herbert and Jean Lapierre that they planned to use 1995 Quebec Referendum as a way to advocate for a new partnership between Quebec and Canada. However, Parizeau advocated for separation; so both Bouchard and Dumont followed his direction.^[5] In subsequent years, however he changed positions on the issue, arguing that the sovereignty question had been decided and that Quebecers had no desire to revisit it.

In the years that followed, the constitutional position of the ADQ changed to favour Quebec autonomism, supporting increased powers and responsibilities for Quebec while remaining within Canada. In practical terms, Dumont has supported the creation of a Quebec constitution, the change of the province's name to that of the "autonomous state of Québec", and the collection of all taxes by the provincial government, with the funds necessary for the federal government to fulfill its constitutional responsibilities being transferred to it by the province.

Since the 1998 provincial election, Dumont has distinguished himself primarily for his stances on economic issues, which are generally considered to be right of centre in the context of the predominantly social-democratic Quebec political scene. He advocates the payment of a cash allowance to parents who do not wish to make use of the province's $7 a day daycare program, changes to the provincial health system that would allow private for profit health care and private health insurance alongside the public system, the abolition of school boards, the encouragement of private education, a tougher criminal justice system, reductions in the size of the provincial bureaucracy, and the repayment of the province's $127 billion government debt.

Dumont was personally very popular, and for most of his career was far more popular than the ADQ as a whole. In fact, for many years, the ADQ tried to capitalize on Dumont's personal popularity by using the official name Action démocratique du Québec-Équipe Mario Dumont (Action démocratique du Québec-Team Mario Dumont). However, this didn't translate to support for his party; it never won much support in Montreal or the Outaouais.