In 1919, Casson moved to Rous and Mann Ltd, where he was influenced by and assistant to Group of Seven member Franklin Carmichael to sketch and paint on his own.

Carmichael and Casson then moved on to the first Canadian silkscreen printing firm, Sampson-Matthews Ltd, founded by artist J. E. Sampson and businessman C. A. G. Matthews,[4] for which he worked from 1926 till 1958.

He also was helpful as a special consultant to the Anti-Rackets Branch of the Ontario Provincial Police uncovering forgeries of Tom Thomson and the Group of Seven and many other historical Canadian artists.

[11][12] A. J. Casson died on February 20, 1992, just three months short of his 94th birthday, and is buried on the grounds of the McMichael Canadian Art Collection, along with six other Group of Seven members.

[17] A large oil on canvas by the Group of Seven's A. J. Casson, Street in Glen Williams, sold for a record $542,800, including buyer's premium on June 1, 2010.

Canadian art critic and historian Paul Duval wrote in 1980, "Street in Glen Williams is unquestionably his key autumn portrayal."

[18] On Wednesday, November 23, 2016, his Gathering Storm sold for $1,534,000 CDN (premium included) – the highest amount paid for a Casson painting to date.

[19] At the Heffel Auction of Canadian, Impressionist & Modern Art, December 1, 2021, Casson's Pic Island, Lake Superior, an oil on board sketch, 9 3/8 x 11 1/8 in, 23.8 x 28.3 cm, estimated at $40,000 - $60,000, sold for: $481,250 (including Buyer's Premium).