He was educated at Stowe and Trinity College, Cambridge, where he gained a First in Modern Languages.
He entered the Foreign Office in 1938, completing the entrance examinations at the top of his intake. He was appointed Third Secretary that year. Following the outbreak of the Second World War, Henniker was released for military service.
He was commissioned as an officer into the Rifle Brigade, eventually rising to the rank of Major. During the war he served in the Western Desert Campaign, being wounded in Libya and hospitalised in Cairo. In 1943 he joined Fitzroy Maclean's mission in Yugoslavia whilst on secondment to the Special Operations Executive.
In 1945 he was awarded the Military Cross. After the war he returned to the diplomatic service and served at the British Embassy in Belgrade from 1945 to 1946, as Assistant Private Secretary to the Foreign Secretary Ernest Bevin from 1946 to 1948, at the Foreign Office from 1948 to 1950 and at the British Embassy in Buenos Aires from 1950 to 1952. From 1953 to 1960 he was Head of the Personnel Department at the Foreign Office.
In 1960 Henniker was appointed Ambassador to Jordan, a post he held until 1962, and was then Ambassador to Denmark from 1962 to 1966. He served as Assistant Under-Secretary of State from 1966 to 1967 but declined the ambassadorships to Brazil and the Republic of Ireland as he wanted to stay in England. In 1968 he became Director-General of the British Council, which he remained until 1972, before being invited to resign by Lord Fulton on the pretext of his wife's failing health.
Henniker devoted the later part of his life to charitable causes, especially in Suffolk where his family had their ancestral seat. In 1980 he succeeded as eighth Baron Henniker on the death of his 97-year-old father, and assumed his seat in the House of Lords. He was made a CMG in 1956, a CVO in 1960 and a KCMG in 1965.
Lord Henniker married, firstly, Margaret Osla, daughter of James William Benning, in 1946. They had three children:
Mark Ian Philip Chandos Henniker-Major, 9th Baron Henniker (born 29 September 1947), married Lesley Antoinette Foskett
Hon. Charles John Giles Henniker-Major (2 September 1949 - 9 May 2012), married Sally Kemp Newby
Jane Elizabeth Henniker-Major (born 6 July 1954), married Richard Spring, Baron Risby.
In the Lords, he briefly served as a spokesman for the Liberal Democrats.