Justice Guy Cournoyer earned a Bachelor of Laws in 1986 from the Université du Québec à Montréal and was admitted to the Quebec Bar in 1987.
He was a partner at the firms Poupart et Cournoyer and Shadley Battista, s.e.n.c., where he practised criminal law, disciplinary and professional law and military law.
He was special counsel to three public commissions of inquiry: the Commission of Inquiry into Certain Events at the Prison for Women in Kingston presided by the Honourable Madam Justice Louise Arbour in 1995-1996; the Commission d’enquête chargée de faire enquête sur la Sûreté du Québec presided by the Honourable Mr. Justice Lawrence Poitras, Q.C. in 1997-1998; and, in 2004-2005, the Commission of Inquiry into the Sponsorship Program and Advertising Activities presided by the Honourable Mr. Justice John H. Gomery.
From 1995-1996, he was the first president of the Association québécoise des avocats et des avocates de la défense (AQAAD).
He is also the author of several publications in criminal, penal, military and disciplinary law, including the Code criminel annoté – Cournoyer-Ouimet and the Code de procédure pénale du Québec annoté.
He was appointed to the Quebec Superior Court (Montreal) in 2007 and to the Court Martial Appeal Court of Canada on July 30, 2009.
Justice Cournoyer was appointed to the Court of Appeal of Quebec on September 30, 2020.