Justice Guy Gagnon was born in Amos in 1953. He attended college in Rouyn-Noranda and then earned his Licentiate in Laws from the University of Ottawa in 1977. He was called to the Quebec Bar the following year.
From 1978 to 1992, he practiced law in a private firm. On February 6, 1992, he was appointed to the Court of Québec where he served in the civil division, the criminal and penal division as well as the youth division. He also served as a member of the circuit court for James Bay, Hudson Bay and Ungava Bay.
In September 2003, he became Chief Judge of the Court of Québec and thus Chairman of Quebec's judicial council, roles he would continue to fill for six years. He was appointed to the Court of Appeal of Quebec on September 9, 2006.