George Mitchell Croil was born on June 5, 1893 in Milwaukee, Wisconsin, United States of America. His parents, Thomas Croil and Christian Mitchell, were Scottish immigrants to the United States. George Mitchell Croil was a first cousin of US Brigadier General Billy Mitchell, a pioneer of American military aviation.
When Croil was 11 years old, he moved, with his parents, to Montreal in Quebec where he attended the Westmount Academy from 1903 to 1907. From 1907 to 1911, Croil lived in Scotland, studying at Robert Gordon's College in Aberdeen. Remaining in Great Britain, Croil studied Civil Engineering for the next two years.
In 1912 Croil moved to Ratnapura, Ceylon where he gained employment as an assistant superintendent with the Mahawale Tea and Rubber Estate. His duties included overseeing the work routines of 700 plantation workers and ensuring that the machinery was kept running.
With the outbreak of World War I, Croil enlisted in the Gordon Highlanders as a private soldier. He was soon commissioned, serving as a machine gun officer in the 5th Battalion of the Gordon Highlanders. In January 1915 he received an acting promotion to captain and took up duties as a company commander.
In May 1916, Croil was detached to the Royal Flying Corps and undertook a two-month period of flying training. 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.
Croil became a member of the Canadian Air Board in June 1920 and played a key role in the establishment of air bases at Morley and High River in Alberta. On the establishment of the Royal Canadian Air Force in 1924, Croil was one its founder officers. Although the RCAF was described as an air force and had a separate rank structure in line with the British air force, the RCAF was under the authority of the Canadian Army. The following year, in 1925, Croil was posted to Great Britain where he served as a liaison officer with the Royal Air Force. Croil also took a course of training at the RAF Staff College, Andover.
Returning to Canada by 1928, Croil was appointed Station Commander of RCAF Station Borden in Ontario which at that time was one of a very few RCAF training bases. Returning to Great Britain for further advanced training, Croil attended the Imperial Defence College. Crossing back across the Atlantic once more, in 1934 Croil was appointed Senior Air Officer with the RCAF making him the head of the RCAF. In 1938, Croil succeeded in obtaining the RCAF's independence from the Army and accordingly his post was upgraded to Chief of the Air Staff. Croil then only reported to the Minister of National Defence.
Croil remained as Chief of the Air Staff until 1940 when he was replaced. He then served as Inspector-General of the Royal Canadian Air Force until 1944.
Croil died on April 8, 1959, in Vancouver, British Columbia.