Rishi Sunak has vowed to provide "stability and unity" after weeks of Tory-inflicted chaos in an 86-second speech.
The new Conservative leader said he was "humbled and honoured" to get the backing of Tory MPs, who handed him the keys to No10.
In a televised address, he paid tribute to his crisis-hit predecessor Liz Truss, and described her 7-week stint in Downing Street as leading with "dignity and grace through a time of great change".
Mr Sunak, who becomes the first British Asian Prime Minister, said: "It is the greatest privilege of my life to be able to serve the party I live and to give back to the country I owe so much to.
“The United Kingdom is a great country. But there is no doubt we face a profound economic challenge.
“We now need stability and unity and I will make it my utmost priority to bring our party and our country together.
“Because that is the only way we will overcome the challenges we face and build a better, more prosperous future for our children and our grandchildren.
“I pledge that I will serve you with integrity and humility - and I will work day in, day out to deliver for the British people.”
Once the brief speech ended, Mr Sunak sidled off the stage awkwardly.
He broke cover this afternoon after conducting the entire four-day contest without speaking publicly, doing any interviews or spelling out his policy agenda.
His first speech after being named as the new PM was to Tory MPs, behind closed doors in Westminster. He then travelled to Conservative HQ in London to meet supporters and give a live address to the nation.
He has kept an ultra-low profile in recent weeks after losing to Liz Truss in the summer leadership race.
Mr Sunak was not at the Conservative party conference earlier this month when the wheels really began to come Ms Truss's premiership. An ally told the Sunday Times that he wanted to give her the space to "own the moment".
But Ms Truss's resignation last week offered him another chance at the Tory crown, which he secured today without a full contest.
His rival Penny Mordaunt failed to get the 100 MP backers needed to proceed to the vote of members. Her campaign said she missed out by a handful of votes with 90 supporters.
Boris Johnson pulled out of the race late on Sunday night - despite jetting back from a Caribbean holiday to try to convince Tories he was the only one to save them from electoral wipeout.
Mr Sunak is not expected to enter Downing Street until tomorrow, as the handover of power is still being hammered out, No10 suggested.
Outgoing Prime Minister Liz Truss must offer King Charles her formal resignation before a new Prime Minister can be confirmed.
Mr Sunak will go to Buckingham Palace to meet the monarch, who will ask him to form a Government in his name.
Downing Street confirmed the transition from Ms Truss to Mr Sunak “won’t be taking place today”, as discussions are still under way with Buckingham Palace.
Updated timings are expected to be released later this afternoon.
Ms Truss is likely to give a farewell speech in Downing Street before going to see the King to resign. Rishi Sunak would then go to the Palace shortly afterwards to be confirmed as PM by King Charles.
No10 was unable to immediately guarantee whether it would be Liz Truss or Rishi Sunak giving PMQs on Wednesday, though it is widely expected to be Mr Sunak.
A No10 spokesman said they would “make sure we have a smooth and efficient process as we did previously”.
The spokesman refused to rule out the possibility of her awarding a slew of resignation honours - despite being in office less than 50 days.
