James Winslow Mortimer (nicknamed "Win") was born in Hamilton, Ontario, Canada in 1919. His father, who work in the poster department of a lithography company, trained him in being an artist from an early age. Upon graduating high school, Mortimer continued his graphic education at the Art Students League of New York. When World War II erupted, he enlisted in the Canadian Army but returned to civilian life in 1943. He then found work designing posters, following his father's example, until being contracted to become an illustrator by DC Comics in 1945.