Early diplomatic career Sedwill joined the
Foreign and Commonwealth Office (FCO) in 1989 and he served in the Security Coordination Department and the
Gulf War Emergency Unit until 1991. He was then posted in
Cairo,
Egypt, from 1991 to 1994 as a Second Secretary, then First Secretary in Iraq from 1996 to 1997 whilst serving as a
United Nations weapons inspector, then in
Nicosia,
Cyprus, as First Secretary for Political-Military Affairs and Counterterrorism from 1997 to 1999. He was the Private Secretary to the
Secretary of State for Foreign and Commonwealth Affairs (
Robin Cook and later
Jack Straw) from 2000 to 2002 in the run-up to and preparations for the 2003
Iraq invasion. He was succeeded as ambassador temporarily by his predecessor, Cowper-Coles, and then by
William Patey, formerly
British Ambassador to Saudi Arabia.
Home Office and National Security Adviser In February 2013, Sedwill became the
Permanent Secretary at the Home Office, filling the vacancy left by
Helen Ghosh. Sedwill replaced
Mark Lyall Grant as National Security Adviser in the Cabinet Office in April 2017. During his time as Permanent Secretary, one of the organisations the Home Office is responsible for,
MI5, failed to adequately safeguard data. In 2019 Lord Justice Sir
Adrian Fulford stated MI5 had a "historical lack of compliance" with sections of the
Investigatory Powers Act in 2016.
Cabinet Secretary in March 2019. Sedwill became acting Cabinet Secretary in June 2018, while
Jeremy Heywood took a leave of absence on medical grounds, and was appointed to replace Heywood on his retirement on 24 October 2018. He is the second Cabinet Secretary never to have worked at
HM Treasury, and the first whose career has been dominated by diplomatic and security work. He was described as the Prime Minister's "first and only choice" to replace Heywood, with no recruitment process taking place and some suggesting the urgency of arrangements for the UK's departure from the European Union as a reason for the quick appointment. Prime Minister
Theresa May was criticised for allowing Sedwill to remain as National Security Adviser alongside his role as Cabinet Secretary, with speculation that the role was being kept for Europe adviser
Oliver Robbins. In a February 2019 interview Sedwill said he would retain his role as National Security Adviser to the Prime Minister since becoming Cabinet Secretary is part of moves to make a success of
Brexit. In an interview with
Civil Service Quarterly, Sedwill said retaining the post would also ensure a "genuine sense of teamwork across and beyond government". In April 2019 it was reported that Sedwill had written to ministers on the
National Security Council and their special advisers after
The Daily Telegraph reported details of a meeting about Chinese telecoms company
Huawei. Following the meeting of the council, the
Telegraph reported that it had agreed to allow Huawei limited access to help build Britain's new
5G network, amid warnings about possible risks to national security. Several cabinet ministers have denied they were involved. In July 2019,
The Times reported that two unnamed senior civil servants had said the 70-year-old
Jeremy Corbyn might have to stand down due to health issues. The article drew an angry response from Labour, which denounced the comments as a "scurrilous" attempt to undermine the party's efforts to gain power. Downing Street said that Sedwill would write to Corbyn after the party demanded an inquiry into alleged comments. Corbyn said the civil service has a duty to be non-political. In November 2019, Sedwill blocked the publication of a document written by civil servants to cost the Labour Party's fiscal plans before a general election.
Shadow Chancellor John McDonnell had complained to Treasury Permanent Secretary
Tom Scholar in a meeting arguing it would interfere in the upcoming general election. In June 2020, it was announced that Sedwill would be stepping down from his civil service appointments in September 2020.
The Telegraph said that Downing Street regarded Sedwill as "too much of a Europhile and establishment figure" to be in post through planned Whitehall reforms. Sedwill stepped down as national security adviser in September 2020. He was to be replaced by
David Frost, who was Johnson's
special adviser and chief negotiator in talks on the post-Brexit trade and security relationship with the EU. This would be a political appointment, while all previous national security advisers had been civil servants. However, an FOI answer stated that Frost continued as Chief Negotiator to the EU as of October 2020, and
David Quarrey took over the role of acting NSA. Sedwill was replaced as Cabinet Secretary and
Head of the Home Civil Service by
Simon Case on 9 September 2020.
Later career Sedwill is a senior adviser and supervisory board member of
Rothschild & Co, senior adviser and member of
McKinsey & Company’s Geopolitics Advisory Council, and sits on the board of directors of
Hakluyt & Company. He is also the chairman of the Atlantic Future Forum and chairman of the trustees of the
International Institute for Strategic Studies. He was previously a non-executive director of
BAE Systems plc between 2022 and 2024, and was the senior independent director and senior deputy chair of
Lloyd's of London between 2021 and 2025. Speaking to the Covid Inquiry,
Simon Case and Sedwill described
Boris Johnson’s Downing Street as “poisonous”, “mad” and unfit to run the country. Giving evidence, “I’ve never seen a bunch of people less well-equipped to run a country,” Simon Case, then head official in the Cabinet Office, wrote in July 2020 to Mark Sedwill. In October 2025, Sedwill was heavily rumoured to be the frontrunner to replace
Peter Mandelson as
British Ambassador to the United States, though the role ultimately went to
Christian Turner. Sedwill has served as a non-executive member of the
Court of the University of St Andrews. He was a candidate in the
2026 election for the position of
Chancellor of the University of St Andrews, which was won by Dame
Anne Pringle. == Personal life ==