Little is known about Sir John's previous work at MI6
Sir John Sawers has taken up his new role as head of the UK's intelligence service MI6, bringing Sir John Scarlett's five-year tenure to an end.
Most recently the UK's permanent representative to the UN, Sir John takes over at a time when the actions of MI6 are under the microscope like never before.
Police are probing claims its agents were complicit in the torture of terrorist suspects overseas, while Parliament's Joint Human Rights Committee has also called for an independent inquiry into a "disturbing number of credible allegations" against agents from both MI6 and domestic security service MI5.
So, among Sir John Sawers' most pressing tasks will be ensuring that strict new guidelines on the questioning of terror suspects are enforced.
His new high-profile may also take some adjustment.
In a far cry from the days when Britain was in the grip of Cold War tension and the head of its Secret Intelligence Service was known only as "C", these days the public not only know Sir John's name but his preferred brand of swimwear (Speedo, according to Foreign Secretary David Miliband).
Indeed, the hasty withdrawal of his personal details from Facebook, which featured holiday snaps of him on a beach posted by his wife Shelley, marked an inauspicious welcome to life at the head of MI6 after his appointment was announced in June.
But such embarrassment has not been a regular occurrence for a man known for his sound judgement and slick delivery when dealing with journalists.
Indeed, he is said to cut a deft figure on the dance floor and his suave manner has led to him being likened by the press to James Bond actor Pierce Brosnan.
Born in Warwick in 1955, Sir John attended school in Bath before studying physics and philosophy at Nottingham.
A keen academic, he also attended the universities of St Andrews, Witwatersrand in South Africa and Harvard in the US.
Sir John has top-level contacts, such as US Secretary of State Hillary Clinton
Announcing his appointment, Downing Street referred to him "rejoining" MI6 but gave no further details of his former career as a spy which reportedly began in the late 1970s.
Nevertheless, giving the job to someone more recently associated with diplomatic service was regarded as a break from the tradition.
As Britain's permanent representative to the UN since 2007, Sir John has built a strong relationship with Secretary General Ban Ki-moon.
He became adept at delivering sound bites summing up both the issue and Britain's position, and played a central role in negotiating and drafting resolutions on Iran, North Korea and the Middle East.
Previously Sir John had been political director at the Foreign Office, an envoy in Baghdad and also a foreign affairs adviser to then prime minister Tony Blair.
Between 1999 and 2001 he was involved in the Kosovo conflict and Northern Ireland peace process.
He has also worked in the British Embassy in Washington, as an ambassador to Cairo and in South Africa from 1988 and 1991 as apartheid was ending.
An athletic sort, the father of three grown-up children still holds his school's 400m hurdles record. He maintains his fitness through hiking and sports such as tennis and cycling.
Not a typical diplomat, Sir John was the only ambassador to visit a camp for people displaced by the fighting in Darfur, Sudan, while his peers were criticised for not seeing how they were forced to live.
After his hands-on role in foreign affairs, he will now have to lead from the front in the struggle to stop global terrorism at a time when treatment of detainees is under intense scrutiny.