He joined the Royal Air Force Reserve of Officers in May 1937. Called after the declaration of war, he joined No. 615 (County of Surrey) Squadron on 11 September 1939. His passage was short-lived as he was transferred at No.152 (RAF) Squadron on 2 October. He is, however, quickly detached to the Defense Flight of Sumburgh, renamed Fighter Flight Sumburgh then No.247 (RAF) Squadron. He later joined the No.313 (RAF) Squadron as B Flight Commander between 3 June and 15 September 1941. He survived the war and left the Royal Air Force with the rank of Wing Commander in 1948. He died in 1969.
Battle of Britain London Monument – Flying Officer T.W. Gillen : http://www.bbm.org.uk/Gillen.htm [Consulté le 8 avril 2016] ; Biplane Fighter Aces from the Second World War – Wing Commander Thomas William Gillen : http://surfcity.kund.dalnet.se/commonwealth_gillen.htm [Consulté le 8 avril 2016] ; London Gazette (The) – 24 august 1937 : https://www.thegazette.co.uk/London/issue/34429/page/5389 [Consulté le 8 avril 2016] ; London Gazette – 6 january 1959 : https://www.thegazette.co.uk/London/issue/41598/supplement/160[Consulté le 8 avril 2016].