Peter Westmacott was born on 23 Dec 1950, the son of the Rev. Prebendary Ian Field Westmacott and of Rosemary Patricia Spencer Westmacott. He was educated at Taunton School and New College, University of Oxford, and in 1972 married (1st), Angela Margaret Lugg (marriage dissolved in 1999), having two sons and one daughter; and then married (2nd) in 2001, Susan Nemazee.

Westmacott entered the Foreign and Commonwealth Office in 1972 and in his diplomatic career served in the following posts: Tehran [Iran] 1974, Brussels [Belgium], 1978; First Secretary, Paris, 1980; Private Secretary to Minister of State, FCO, 1984; Head of Chancery, Ankara, 1987; Deputy Private Secretary to Prince of Wales, 1990; Counsellor, Washington, 1993; Director, Americas, FCO, 1997; Deputy Under-Secretary of State, FCO, 2000-01; Ambassador to Turkey, 2002-06; Ambassador to France, 2007-11; Ambassador to United States, 2012-15.