By 1917, he was promoted to flight lieutenant and by March 1918 to major, commanding the Naval Airship Patrol Station on Anglesey in Wales.
He was then second-in-command of British Air Forces in North West Europe until the end of the war,[6] serving in D-Day, Normandy, the Ardennes and the advance across the France and Germany.
[6] As independence approached, Pandit Nehru asked him to be the first Commander-in-Chief of the Royal Indian Air Force in the new Union of India.
[6] In 1953, Elmhirst ran Operation Totem, the first British nuclear bomb land tests in Emu Field, Australia.
Elmhirst married firstly Katherine Gordon Black, daughter of William Black, on 16 December 1930,[7] and had two children before Katherine's death in 1965: On 30 October 1968, he married Marian Ferguson (née Montagu Douglas Scott), widow of Colonel Andrew Henry Ferguson.
Marian was the daughter of Lord Herbert Montagu Douglas Scott and Marie Josephine Edwards, and the granddaughter of The 6th Duke of Buccleuch and Lady Louisa Hamilton.