[2][5] He distinguished himself as a marksman, representing Canada on the Bisley Team on three occasions and receiving several medals and prizes for his marksmanship.

By 1914, when World War I began, Rennie was a lieutenant colonel, and commanded the third battalion of the Canadian Expeditionary Force.

During the war he was promoted to brigadier general, commanded the 4th Canadian Brigade of the 2nd Canadian Division, and was promoted to major general after the war on September 16, 1921 Robert ran unsuccessfully as a candidate for the Liberal Party of Canada, for the Federal riding of North Toronto after he stepped down from the family firm.

Thomas and Robert Rennie filed objections that were couched in legal principles but were actually based, according to the lawyer for the Rennie brothers, upon the notion that people of foreign descent should not be allowed to assume the proud Scottish name of Rennie.Robert had been a stamp collector, while at the family firm.

When the value of his portfolio dropped, following the 1929 stock market crash, Robert gradually liquidated his extensive collection of valuable stamps.