Josh Sumner was born on January 28, 1906. He graduated from the University of Manitoba in 1926 with a Bachelor of Science in Civil Engineering.
Sumner worked for the Royal Canadian Air Force and then, in 1946, he began working for the architecture firm Green Blankstein Russell (GBR). Sumner acted as director of the engineering and provincial offices of GBR and sat on the design committee alongside GBR architects such as David Thordarson and G. L. Russell. Sumner also did many of the estimates for the firm.
Sumner was a member of the Associations of Professional Engineers in Manitoba and Alberta, the Engineering Institute of Canada, the Association of Consulting Engineers of Canada Inc., the Association of Professional Engineers of Canada and the Appraisal Institute of Canada.