W/Cmdr G. L. Sinclair joined 19 Squadron on 27th November, 1937.
He was still with the squadron and saw action over Dunkirk destroying a number of aircraft. He was awarded the DFC (25/6/40). In late June he was posted to 310 (Czech) Squadron as A Flight Commander and claimed 4 more victories. On 9th September, he was involved in a collision with another aircraft and forced to bale out. 
On 29th September, he was shot down by a Bf 109 in an engagement but baled out unhurt. He was awarded the Czech Military Cross (24/12/40).

In January, 1941, he was posted to HQ 12 Group on staff duties. He then formed and commanded 313 Squadron in May, 1941.
He took command of 79 Squadron in September and led it until posted to Air Ministry in December. He was given command of 56 Squadron at Bradwell Bay in October 1943 and was promoted to Wing Commander in May 1944 and joined the staff at HQ 84 Group.

In January 1945 he returned to the Air Ministry and in October was appointed Personal Air Secretary to the Secretary of State for Air. He was also awarded the Order of King George of Podograd by the Czechs in recognition of his wartime efforts.