Longmore joined the
Royal Naval Air Service in 1912. Before the outbreak of the
First World War he served as a flight instructor at the newly formed
Central Flying School, Upavon, and was then appointed commander of the seaplane base at
Cromarty and later of the
experimental seaplane establishment at Calshot. During the war he served as
Officer Commanding No. 3 Squadron RNAS and then as Officer Commanding
No. 1 Squadron RNAS before being transferred back to sea duties as an officer on the
battlecruiser in 1916; during his service aboard
Tiger he took part in the
Battle of Jutland. He obtained a permanent commission in the
Royal Air Force in 1920 and was appointed Air Officer Commanding
No. 3 Group later that year before being given command of the RAF Depot in 1921. He was made Air Officer Commanding
No. 7 Group in 1924, Director of Equipment at the
Air Ministry in 1925 and Chief Staff Officer at Headquarters Inland Area in 1929. Subsequent appointments included Commandant of the
Royal Air Force College Cranwell in December 1929, Air Officer Commanding Inland Area in 1933 and Air Officer Commanding Coastal Area (which was renamed
RAF Coastal Command under his leadership) in 1934. He went on to be
Commandant of the Imperial Defence College in 1936. The outbreak of the
Second World War found Longmore an
air chief marshal and in charge of
RAF Training Command. On 2 April 1940, he was appointed
Air Officer Commanding in the
Middle East. He did not long enjoy the full confidence of
Winston Churchill in that position and was relieved of his command in May 1941. His last role before his formal retirement in 1942 was as
Inspector-General of the RAF. The
Oxford Dictionary of National Biography says the following of Longmore during the first few months of 1941: :Longmore's constant demands for reinforcements resulted in some unwelcome attention from Churchill, who hated pessimists and senior commanders who complained about their lack of resources. After some acerbic correspondence, in which Churchill accused Longmore of failing to make proper use of the manpower and aircraft he had, Longmore was recalled to London in May 1941. He was succeeded in the Middle East by Air Chief Marshal Sir Arthur Tedder. In retirement he was vice-chairman of the
Commonwealth War Graves Commission. Longmore's memoirs,
From Sea to Sky 1910–1945, were published in 1946. ==Grantham by-election, 1942==