Deacon was born and raised in a family of ten children in Toronto, Ontario and attended the University of Toronto Schools. In 1942, he volunteered to serve in the 3rd Medium Regiment, Royal Canadian Artillery during World War II, rising to the rank of Captain. Deacon, a Forward Observation Officer, travelled with front line infantry in order to direct artillery fire via radio transmission. In later years, he often commented at how accurately George G. Blackburn had captured the role of a FOO in his book "The Guns of Normandy".