Selwyn Hanington Dewdney was born on October 22, 1909, in Prince Albert, Saskatchewan.

He was a Canadian writer, illustrator, artist, activist and pioneer in art therapy and pictography. He graduated from the University of Toronto (B.A. in astronomy and English), the Ontario College of Education (1932) and Ontario College of Art (1934). In 1928, he accompanied his father on a 3,800 mile canoe journey to visit the Ojibway and Cree missions in Northern Ontario. This experience established his interest in native culture and love of the bush in the Canadian Shield. He taught at Sir Adam Beck Secondary School, London, Ontario. With a growing family of three sons, he turned to illustrating books, writing, researching, editing and painting commissioned murals to support them. When Dewdney was commissioned to illustrate Lionel Penrose's psychiatric "M" test, he became interested in art therapy. He began giving art instruction to the psychiatric patients, which led to his position of Psychiatric Art Therapist. His and his wife’s work helped develop an art therapy training program at the University of Western Ontario in 1986. Dewdney helped record the ancient native pictographs painted in red ochre on the rocks. By 1957, eleven rock-painting sites were recorded in Quetico Provincial Park. Between 1959 and 1965, he discovered and recorded rock art from the foothills of the Rockies to the Atlantic coast. He is the author of the books "Wind Without Rain" (1946) and "Christopher Breton" (1978).

In 1936, he married Irene Maud Donner (1914–1999). He died on November 18, 1979, in Toronto, Ontario, following an open heart surgery.