He served as the British Ambassador to the United States between January 2016 and December 2019, and previously as National Security Adviser and UK Permanent Representative to the European Union.

[2][7] He served in a number of posts, including as desk officer for the Channel Tunnel project and co-secretary of the UK-French Channel Tunnel Treaty Group, as private secretary to David Mellor and then The Lord Glenarthur as the FCO's Minister of State from 1987 to 1989, and as Counsellor for External Affairs at the British Permanent Representative to the European Union for a year before being promoted to Director as head of the FCO's press office in 1998.

[2] In 2000, Darroch moved back to policy work as Director of EU Comd,[clarification needed] and in 2003 promoted further to be Director-General, Europe.

After three years, Darroch was appointed to replace John Grant in Brussels, as British Permanent Representative to the European Union in 2007 for a four-year term.

[9] On 20 August 2015, the Foreign Office announced that Darroch's new role would be as the Ambassador to the United States, replacing Peter Westmacott on 28 January 2016.

[10][11] In November 2016, following the US election, a memo by Darroch to Prime Minister Theresa May was leaked in which he said the President-elect of the United States, Donald Trump, could be influenced by the British government.

[14] On 7 July 2019, secret diplomatic cables from Darroch to the British government, dating from 2017 to 2019, were leaked to Steven Edginton, a 19-year-old freelance journalist and Brexit Party employee.

[21] In the House of Commons, both May and the leader of the opposition, Jeremy Corbyn, praised Darroch's service and deplored that he had to resign under pressure from the United States.