Clarence Alphonse Gagnon was born in Montreal November 8, 1881. From 1897 to 1900, he studied drawing and painting under William Brymner at the Art Association of Montréal. Gagnon left for Paris in 1904 to work in the studio of Jean-Paul Laurens at the Julian Academy.
He won a gold medal at the St. Louis Exhibition in 1904 and an honourable mention at the Salon des artistes français in Paris in 1905 for his engravings.
From 1909 to 1914 Gagnon moved between Canada, France, and Norway. He often painted scenes of The Laurentians in Québec.
Gagnon became a member of the Royal Society of Canada. In 1910, he became an associate member of the Royal Canadian Academy of Arts and a full member in 1922. In 1923, he received the Trevor Prize of the Salmagundi Club of New York. He received an honorary doctorate from The Université de Montréal.
Between 1924 and 1936, Gagnon spent time in Paris and traveled throughout Europe. During this time he illustrated a number of books.
He returned to Québec in 1936.
Gagnon died in Montreal in 1942.