Philip D. McKenna was born 4 October 1895 in Iona, Prince Edward Island to parents Michael McKenna (1899-1929) and Mary Rooney (1852-1916). Philip was one of nine children and his siblings included Peter J., Catherine J., Michael, Owen J., John P., James F., Patrick J., William I., and Annie L. Philip likely attended school in Vernon Bridge and appears to have gone on to study at St. Dunstan's University in Charlottetown.

Following the outbreak of World War One in 1914, many young Islanders enlisted to fight in the Canadian and British armed forces. In July 1916 Philip signed up and on 25 September he left PEI to join the fight overseas. Philip served as a gunner in the 8th and 11th Canadian Siege Batteries. He was stationed in Whitely Camp, England and Dublin, Ireland before being transferred to Fleury Wood, near Bouverie, France. Following the end of hostilities in November 1918, Philip remained in the service until at least April 1919.