Simon Case will take over as the UK’s top civil servant next week after being appointed by Boris Johnson.
Reports surfaced over the weekend that the former private secretary to the Duke of Cambridge was set for the job.
But it was confirmed on Tuesday, with the PM saying Mr Case was “ideally suited for this crucial role”.
It comes after Sir Mark Sedwill announced in June he would stand down following reports of tensions between him and members of the PM’s team.
Mr Case, 41, was made permanent secretary at No 10 earlier this year and he has been a civil servant since 2006.
He spent almost two years working as Prince William’s right-hand man before temporarily moving to Downing Street to assist with the coronavirus response.
Now he will become both the head of the Civil Service and the new cabinet secretary, tasked with advising the prime minister on implementing policy and the conduct of government.
In a statement, Mr Case said it was “an honour” to be appointed, adding: “Over these few months of working on the Covid response, I have seen how much hard work is being done by the Civil Service to support the government and our country through unprecedented times.
“It is a privilege to come into this role to lead a service that is working day in, day out to deliver for people right across the country.”
Former cabinet secretary Lord Gus O’Donnell said Mr Case “knows how to make things happen” and had “clearly developed the trust of the prime minister”.
He told BBC Radio 4’s Today programme it was “a surprise to have someone so young” in the role, but said the new top civil servant had plenty of Downing Street experience, adding: “I’m sure he will grow into the role.”
Lord O’Donnell, who served in the role between 2005 and 2011, said one of Mr Case’s tasks would be getting the prime minister to “understand he needs to work with the civil service rather than against them”.
Some opposition parties have claimed the appointment is political, with Labour’s deputy leader, Angela Rayner, tweeting: “[The] problem with ‘close allies’ or some might say ‘cronies’ is that they often tend to say what you want to hear.”
But Tory Former Cabinet Office Minister David Lidington said Mr Case would “do his utmost for whoever is the elected” and that he was “nobody’s patsy”.
He said the new cabinet secretary was “unafraid to give ministers candid advice, even if it’s unwelcome”.
His appointment comes two months after Sir Mark announced he was planning to step down from the role.
In a letter to Prime Minister Boris Johnson, Sir Mark said it was the right time to go as the government moved to the next phase of its coronavirus recovery plan.
Sir Mark is also stepping down as national security adviser to the PM.
But the job is now being split into a separate role, with the PM’s chief Brexit negotiator David Frost – who is not a civil servant – taking over.
The PM praised Sir Mark on Tuesday for his “outstanding service”, while Mr Case thanked him for his “kindness and support”.
The job will switch hands on 9 December.
The new posting also comes within the wider context of a “fairly radical shake-up” of the civil service, BBC political correspondent Nick Eardley said.
This year has seen a number of senior civil servants in various departments unexpectedly announce they are leaving their posts.
Jonathan Slater, the chief civil servant at the Department for Education, was sacked earlier this month following the row over A-level and GCSE results in England.
He became the fifth permanent secretary to leave his post in six months.
Sir Richard Heaton resigned as permanent secretary at the Ministry of Justice in July, saying it had been “a privilege” to lead at the Ministry of Justice, despite “challenging years”.
Sir Philip Rutnam quit as permanent secretary of the Home Office in February, announcing he would take the Home Secretary Priti Patel to an employment tribunal.
And Sir Simon McDonald announced in June he would step down in as permanent secretary of the Foreign and Commonwealth Office in September “at the request” of the prime minister.