She served as Liberal leader of the Opposition in the National Assembly of Quebec from May 1998 to December 1998 and Deputy Premier in 1994 and from 2003 to 2005.

At the end of the mandate, when Daniel Johnson, Jr. replaced Robert Bourassa as Quebec Premier in 1993, she was named the Minister of Finances, the Deputy Premier and the President of the Treasury Board until the Liberals lost to the Parti Québécois in the 1994 elections.

[citation needed] In the 1998 election, Charest won a seat and replaced Gagnon-Tremblay as leader of the Opposition.

[citation needed] After the Liberals won the 2003 election, Gagnon-Tremblay became deputy premier from May 2003 to February 2005 in the Charest government, and has held various cabinet posts including minister of international relations as well as minister responsible for la francophonie.

Re-elected in the 2007 election, she was renamed the Minister of International Relations, La Francophonie and for the Estrie Region as well as the Vice-Chair of the Treasury Board.