Leahy was educated at Tonbridge School, Clare College, Cambridge and Yale University. After National Service in the R.A.F., Leahy joined the Foreign Office in 1952 at the age of 24. He began his career with a position in the Central Department as "Desk Officer", responsible for dealing with the Soviet zone of post WWII Germany (i.e., "East Germany" and occupied Austria).
In a series of diplomatic appointments, Leahy became assistant private secretary to Selwyn Lloyd, Minister of State, who later became Foreign Secretary during the Suez crisis. Leahy also served as Foreign Office spokesman and was later seconded for a time to the Northern Ireland Office as Under-Secretary of State.
Back at the Foreign Office, Leahy was appointed ambassador to South Africa and subsequently became the FCO's Deputy Under-Secretary of State (DUSS) for Africa and the Middle East. His last diplomatic appointment was as High Commissioner to Australia.
Regarded as having "a safe pair of hands", Leahy carried out a number of sensitive assignments on behalf of the British government, and came face to face with leading political figures of the day. For example, in April 1984, he was sent to Jamba in Angola to secure the release of 16 Britons who had been taken hostage by the Angolan rebel leader, Jonas Savimbi. At the time, Savimbi's UNITA guerrilla movement was financed and supported militarily by the apartheid regime in South Africa.The mission was successful and Leahy brought the captives back to London.
After his retirement from HM Diplomatic Service, Leahy held several non-executive appointments, including being a director of the Observer newspaper, before joining Lonrho as a non-executive director in October 1993. He became Chairman in November 1994. On 2 March 1995, after many internal upheavals, the company board dismissed the controversial businessman (Tiny Rowland), one of the Joint Chief Executives. In 1997 Leahy was succeeded by Sir John Craven.
Life of Spice is a book written by Sir John Leahy, published in 2006. The book includes details of his life prior to becoming a career diplomat. It also extensively includes details of his later appointments and travels as Ambassador to South Africa, Deputy Under-Secretary for Africa and the Middle East (including a specific chapter regarding his position in Tehran, Iran as Head of Chancery), and as High Commissioner in Australia. It contains many original photographs and noteworthy mention of his extensive travels, including both personal and professional aspects of his life. It also contains detailed descriptions regarding some of the operations and conflicts in which he was engaged around the world.
Sir John Leahy was invited as a guest speaker by the School of International Studies (SIS) at the University of the Pacific in Stockton, CA. Marjorie Chase, International Editor for the Pacifican newspaper (1990-1991) took the rare opportunity to personally interview him. She published a brief article about him in the Pacifican. Regarding the interview, she states, "I found him to have been a fascinating man, fully professional, but quite down to earth." As Miss Chase was also educated at a British institution of higher learning (Worcester College, Oxford), she enjoyed sharing an inside understanding of Britain's unique culture. At the time of the interview, Chase was pursuing a degree at U.O.P. in International Affairs and Commerce and was quite involved in news coverage of Middle East affairs of Iran, Iraq, Kuwait and other Middle East countries (especially as his visit was during the height of the Iraq/Kuwait invasion), so she considered this interview to be an exclusive privilege.
Sir John Leahy died on 17 November 2015 at the age of 87.
Leahy and his wife Anne had four children.