Courchesne entered politics in 2003, where she was elected as the MNA for Fabre while the Liberals regained power after nine years of governing by the Parti Québécois. She was named by Jean Charest in the Cabinet as the Minister responsible for the relations with the Citizens and Immigration and was then promoted to Employment and Social Solidarity following a Cabinet shuffle in 2005 where she took the position occupied by Claude Béchard.
Following the 2007 elections, she was re-elected in a Liberal minority government, and named the Minister of Education, Leisure and Sports as well Minister of Family.
Following the 2008 election, she kept most of the portfolios but gave up the Ministry of Family to LaFontaine MNA Tony Tomassi. Courchesne lost the Education portfolio to former Environment Minister Line Beauchamp in a 2010 cabinet shuffle and became President of the Treasury Board. She regained the Education Minister position, as well as becoming Deputy Premier of Quebec, on May 14, 2012, following Beauchamp's resignation during the ongoing student protests over tuition hikes.
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