David Summerhayes was born on 29 September 1922, the son of Sir Christopher Summerhayes, KBE, CMG and Anna (née Johnson). He was educated at Marlborough College and Emmanuel College, Cambridge University. In 1959 he married June van der Hardt Aberson; they had two sons and one daughter.
After serving as a captain in the Royal Artillery during the war, in North Africa and Italy, Summerhayes joined the diplomatic service. His career included: 3rd Secretary, Foreign Office (1948); Baghdad [Iraq], (1949); Brussels [Belgium], (1950-53); 2nd Secretary, FO (1953-56); 1st Secretary (Commercial), The Hague [Netherlands], (1956-59); 1st Secretary and Consul, Reykjavik [Iceland], (1959-61); FO (1961-65); Consul-General and Counsellor, Buenos Aires [Argentina], (1965-70); Head of Arms Control and Disarmament Department, Foreign and Commonwealth Office, (1970-74); Minister, Pretoria/Cape Town [South Africa], (1974-78); Ambassador and Leader, UK Delegation to Committee on Disarmament, Geneva [Switzerland], (1979-82); Disarmament Adviser, FCO, 1983-92.