 He was a member of Phi Delta Theta fraternity and active on the school newspaper.Upon America's declaration of war against Germany on 6 April 1917, in his sophomore year, Rennie withdrew to enlist.He signed into the New York National Guard on 27 April 1917 and was mustered in as a Private in the 29th Company, 8th Regiment, of the United States Army Coast Artillery Corps on 22 July 1917.