While working at the Ottawa-based Conference Board of Canada as an associate economist, Langlois was recruited by newly appointed Parti Québécois finance minister, Pauline Marois.
He was highly involved in the budget preparation and other legislative operations and was a key line writer for the daily question period.
During the 2006 federal election, Bloc Québécois officials asked Langlois to manage Lussier’s campaign against incumbent Liberal minister Jacques Saada.
On April 13, 2012, Pierre Langlois along with Pauline Marois, declared his candidacy for the open seat of La Prairie in the upcoming provincial election for the Parti Québécois.
On September 4, 2012, Pierre Langlois lost by 81 votes against Stephane Le Bouyonnec of the Coalition Avenir Quebec (CAQ) in a close contest.