Glynn served originally in the
10th (Scottish) Battalion,
Liverpool Regiment; he was raised from the
ranks, being promoted from
private to
second lieutenant on 17 November 1914. He was promoted to lieutenant on 4 December 1914, and was appointed an
acting-captain on 12 May 1915, serving as
adjutant until 14 November. He applied for transfer to the
Royal Flying Corps, and on 10 May 1917 was seconded from the Liverpool Regiment to the Royal Flying Corps with the rank of second lieutenant to serve as a
flying officer, having relinquished his acting rank. However, on 31 May 1917 he was promoted to captain in his regiment with precedence from 1 June 1916. After completing pilot training, he was posted to
No. 56 Squadron RFC on 17 June 1917. He did not serve there long, being returned to Britain for additional training. He was reassigned, being posted to
No. 74 Squadron RFC. He returned to
France with this squadron, landing on the
Western Front in early 1918. He scored his first aerial victory with them on 29 April, just after the RFC became part of the RAF. On 1 June he was appointed an acting-captain to serve as a
flight commander in the squadron. Glynn was awarded the Distinguished Flying Cross after his double victory on 1 October 1918, although it was not
gazetted until 3 December 1918. His citation read: ;Distinguished Flying Cross :Lieutenant (Acting-Captain) Clive Beverley Glynn (Liverpool Regiment). ::This officer has shown remarkable skill and bravery in aerial combat, never hesitating to attack, without regard to disparity of numbers. He has accounted for four machines—two in one flight. ==Combat record==