Born in Montreal, Justice Marie St-Pierre is a graduate of Villa-Maria, Collège Jean-de-Brébeuf (D.E.C. 1975) and Université de Montréal (LL.L. 1978).
Admitted to the Quebec Bar in 1979, she spent 23 years working in the Montreal law firms of Blain, Piché, Emery (1979-1989) and Desjardins Ducharme (1989-2002), specializing in civil, administrative and commercial litigation as well as in access to information and protection of privacy.
Over the course of her career, she spent 20 years teaching evidence and procedure, client services, legal writing and advocacy at the École du Barreau. For many years, she also hosted the Quebec Bar's annual Intensive Trial Advocacy Seminar.
Appointed to the Superior Court on May 31, 2002, Justice St-Pierre regularly sat in that court's civil and administrative divisions, among others. She also presided over numerous court committees, including the Civil Procedure Rules Committee.
From January 2008 to October 2010, she presided over the second trial of Widdrington v. Wightman, a judicial saga commonly known as "l'Affaire Castor". From September 2011 until her appointment to the Court of Appeal in April 2012, she acted as the coordinating judge of the Superior Court for the district of Terrebonne.
From 2007 to 2013, she was a teacher and co-chair of the annual intensive for new federally-appointed judges given by the Canadian Institute for the Administration of Justice (CIAJ) and the National Judicial Institute (NJI). Since 2013, she has continued her involvement by teaching trial management and civil evidence.
She was appointed to the Court of Appeal on April 5, 2012.