Croil resigned as CAS in 1940 and then served as Inspector-General of the RCAF until his retirement in 1944, when the post of Inspector General was abolished.

[4] When Croil was 11 years old, his family moved to Canada and settled in Montreal, Quebec, where he attended Westmount Academy from 1903 to 1907.

[4] In 1912 Croil moved to Ratnapura, Ceylon where he gained employment as an assistant superintendent with the Mahawale Tea and Rubber Estate until 1914, where he maintained machinery and supervised workers.

On successfully completing the course and receiving his pilot's wings in July 1916, Croil was seconded from the Gordon Highlanders to the Royal Flying Corps with the temporary rank of captain.

In 1938, Croil succeeded in ensuring the RCAF's separation from the Army, and his post was upgraded to Chief of the Air Staff as a result.