Humphrey was appointed Air Aide-de-Camp to the Queen on 31 March 1974 and Chief of the Air Staff on 1 April 1974. As Chief of the Air Staff he advised the new Labour Government on the implementation of their latest Defence Review. Following promotion to Marshal of the Royal Air Force on 6 August 1976, Humphrey was appointed Chief of the Defence Staff on 24 October 1976. However he only served in that role for only three months before catching pneumonia following a visit to British forces in Norway during a particularly cold Winter. He died in the RAF Hospital at Halton on 24 January 1977.