Macgowan was born in Gowan Brae, PEI, and died in New Westminster, BC. Macgowan was admitted to the bar in 1912, and joined his brother-in-law's firm as a partner. He joined the 131st Battalion of the New Westminster Regiment in 1916. Later he was drafted to the 47th regiment and eventually promoted to the rank of Captain. He was awarded the Military Cross at Vimy, Passchendaele, and Valenciennes. During his wartime service, he was also engaged in scouting missions and providing legal work for army. According to a letter sent to Macgowan on 5 April 1919 from his men, he was held in high regard by the men in his command as a "sympathetic, staunch and loyal friend and adviser."