In December 1914, four months after Britain's entry into the First World War, Glen enlisted in the Royal Scots. He was posted to the Western Front in August 1915 and was lightly wounded in the stomach and hands in June 1916. He won the Military Medal for bravery and was recommended for the Distinguished Conduct Medal on a number of occasions. On 9 April 1917, Glen was serving as a sergeant when he was killed near Arras during the First Battle of the Scarpe. He was buried in Cabaret-Rouge British Cemetery, Souchez.
