Born in Montréal, Québec, Justice de Montigny was educated at the Université de Montreal (LL.L, 1978; LL.M., 1980) and Oxford University (M.Phil, 1981). He was called to the bar of Québec in 1983. Justice de Montigny was a professor at the Faculty of Law of the University of Ottawa (1982-1997) and a lecturer at the École du Barreau du Québec and the faculties of Law and Continuing Education of the Université de Montréal. He acted as counsel in the Constitutional Law Directorate of the Québec Department of Justice (1987-1988) and as Special Advisor to the Québec Department of Intergovernmental Affairs (1992-1993). Justice de Montigny was Director General, Planning and Strategy, at the Privy Council Office from 1997 to 2000. He was subsequently Chief Legal Counsel in the Public Law Group of the Department of Justice of Canada (2000-2003) and Chief of Staff to the Minister of Justice of Canada (January 2004-June 2004). Justice de Montigny was President of the constitutional section of the Canadian Association of Law Teachers (1987-1990) and vice-president of the constitutional section of the Canadian Bar Association (1988-1993). He is the author of numerous books and articles on constitutional law, administrative law and human rights.
Justice de Montigny was appointed Judge of the Federal Court and an ex officio member of the Federal Court of Appeal on November 18, 2004. He was appointed as a judge of the Court Martial Appeal Court of Canada on June 23, 2006. Justice de Montigny was appointed Judge of the Federal Court of Appeal on June 19, 2015.