The descendant of Loyalists, he enjoyed an international reputation as a military historian and analyst, foreseeing the tactical advantages of rapidly deployed mobilized infantry in modern warfare. As Toronto's senior police magistrate 1877-1921, he meted out justice in such a colourful fashion that his court became a tourist attraction.
Commanding the Governor General's Body Guard, a cavalry troop established and supported by his family, he saw action in the 1866 Fenian raid and during the 1885 North-West Resistance. Denison was an active supporter of the Confederacy's operations in Canada during the American Civil War. Afterward, he was a founder of the Canada First movement (1868) and the Canadian National Association, a short-lived political organization (1874).
As the most vocal Canadian spokesman for the idea of imperial unity 1880-1910, Denison portrayed the US as the chief threat to Canadian nationhood, and reasoned that a federation of the British Empire, with Canada as an equal partner, would provide the security, prestige and power essential to national greatness.