He was born in Maple Grove, Quebec and studied with William Brymner in Montreal, going on to study at the Académie Julian in Paris. He served overseas during World War I, taking part in the Somme offensive, and was awarded the Military Cross in 1918. After the war, he worked for Miller Brothers, paper box manufacturers, and became company president in 1921. He left the company to concentrate on painting but had returned to the position of company president by 1926. Hewton also married Isobel Monk (née Robertson) around this time. In 1933, he moved away from Montreal when Miller Brothers moved to Glen Miller, Ontario.
Hewton was a founding member of the Canadian Group of Painters. He was admitted to the Royal Canadian Academy of Arts in 1934. He helped found the Beaver Hall Group, a group of Canadian women painters based in Montreal, in 1920.