Sir Mark Sedwill is to step down as the most powerful official in Government and top civil servant after 30 years in service.
The former diplomat was appointed as Cabinet Secretary in 2018 when he was already the National Security Adviser.
Critics questioned whether the Oxford-educated civil servant would be able to handle the two roles.
Dubbed the ultimate “securocrat”, Sir Mark became the civil service’s highest-ranking official after running the Home Office between 2013 and 2017.
He was a trusted lieutenant of Theresa May when she was PM and sat in her dwindling trusted inner circle after working with her for years at the Home Office.
As Permanent Secretary, Sir Mark oversaw the establishment of the troubled inquiry into the department’s handling of allegations about paedophile activity between the 1970s and 1990s.
He was also forced to tackle fall-out from the termination of a contract to provide the controversial eBorders programme, when a tribunal ruled the Government should pay out more than £220 million to a US defence firm.
In 2018, he played a role in the response to the poisoning of the Skripals in Salisbury.
Known as a keen golfer, windsurfer and scuba-diver, Sir Mark has also been at the centre of Brexit preparations in Whitehall.
In April 2019, the Daily Mail claimed to have gained a copy of a 14-page letter the Cabinet Secretary sent to ministers warning them of the consequences of a no-deal Brexit.
He rose to the defence of the Prime Minister’s Europe adviser Olly Robbins following criticism from Eurosceptics he was thwarting the Brexit process.
Also last year, current Education Secretary Gavin Williamson accused Mrs May and Sir Mark of badly mishandling the investigation into the leak of information from a top-secret meeting of the National Security Council and called for a probe into it.
Mr Williamson was sacked from his role of defence secretary over his alleged involvement in the leak of information about Chinese tech giant Huawei.
Sir Mark was said to be furious at the leak and his investigation was reportedly uncompromising in its dealing with Cabinet ministers and aides, hauling them in for interviews, issuing questionnaires and demanding they hand over mobile phones.
Mr Williamson and his allies claimed his eventual toppling was part of a vendetta pursued against him by Sir Mark.
The pair were reported to have had a series of clashes during Mr Williamson’s time at the Ministry of Defence (MoD).
In an interview with the Civil Service Quarterly in February 2019, Sir Mark admitted he had an “unusual” background for a Cabinet Secretary having spent much of his career overseas.
He joined the Foreign and Commonwealth Office (FCO) in 1989, serving in Egypt, Iraq, Cyprus and Pakistan before becoming ambassador to Afghanistan in 2009.
He later worked as the Nato Senior Civilian Representative in the conflict-hit country before becoming FCO political director in 2012.
Sir Mark studied at St Andrews and Oxford universities and is married with one daughter.
He was knighted in the 2017 New Year honours list for services to national security.