Starting in 1965 and continuing into the early 1970s, he was a representative of the British government in discussions which led to the United Kingdom's entry into the European Economic Community in 1973.

He was a minister at the British Embassy in Washington, DC, in 1984–1985, and was then promoted to ambassador and permanent representative to the European Economic Community from 1985 to 1990.

Hannay took on specialised roles such as United Nations Special Representative for Cyprus between 1996 and 2003 and was a member of the UN High Level Panel on Threats, Challenges and Change, reporting to the Secretary-General in December 2004.

[4] On 19 June 2001 he was created a life peer as Baron Hannay of Chiswick, of Bedford Park in the London Borough of Ealing,[5] sitting as a crossbencher.

He has four sons (Richard, Philip, Jonathan and Alexander) and twelve grandchildren (Peter, Charlotte, Juliette, Edward, Robin, Manon, Harry, Gustavo, Sophie, Julian, Albert and Theodore).