A.J. Casson was the youngest member of the Group of Seven. Born in Toronto in 1898, Casson studied at the Ontario College of Art under J.W. Beatty from 1918 to 1921 and took private classes with Harry Britton from 1916 to 1918. In 1919, he began to work as a commercial artist at Rous & Mann Ltd., where he became the assistant to Group of Seven member Franklin Carmichael, a connection that would prove to be invaluable. Casson joined the Arts and Letters Club in Toronto in 1920, where he met the other Group members. In 1926, Casson was invited to join the Group after the departure of Frank Johnston, and forged his identity among them with his images of southern Ontario villages and rural countryside, as well as the Ontario northland. In this same year, he co-founded the Canadian Society of Painters in Water Colour with Carmichael and F.H. Brigden, to promote the significance of this medium, as well as becoming an Associate of the Royal Canadian Academy (later becoming a full member). He also joined the commercial art firm of Sampson Matthews Limited, where he would work until 1958.

During this time, he explored the Ontario countryside on sketching trips as often as possible, and after his retirement from Sampson Matthews poured all his energy into painting. As well as traveling to such painting places in the wilderness such as Algonquin Park, Haliburton and the Madawaska River, Casson toured southern Ontario towns extensively, painting subjects such as houses, stores and mills, which he imbued with great warmth.
In 1933, he was one of the founding members that created the Canadian Group of Painters. He was also involved with the Ontario Society of Artists, and was elected its President in 1941. As well as exhibiting regularly in Canada with the Group, the Ontario Society of Artists, the RCA, the Canadian Group of Painters and the Canadian Society of Painters in Water Colour, he participated in many international shows such as in Wembley, England, the Jeu de Paume in Paris, the Tate Gallery in London, the Canadian Club in New York and the National Gallery of Art in Washington, to name a few.

Later in life, honours poured in, as his special status as a Group of Seven member garnered great appreciation, and his regular exhibitions at Roberts Gallery sold out. Casson passed away in 1992 at the age of 94.