It caps a meteoric rise for the 42-year-old, who only became an MP seven years ago.
But, while Mr Sunak might allow himself a celebratory can of his beloved Coca-Cola, he will soon have to get to work in No10 with a string of challenges awaiting him.
The parlous state of the economy, the war in Ukraine, possible elections in Northern Ireland, the Channel migrants crisis and reuniting the Conservative Party are all critical tasks lying in his in-tray.
Yet the most immediate duty for Mr Sunak is for him to choose who will sit around the Cabinet table with him in Downing Street.
The incoming PM Rishi Sunak will soon have to get to work in No10 with a string of challenges awaiting him
Penny Mordaunt ended up being Mr Sunak’s closest challenger for the Tory leadership and, by convention, she should be offered a job in the new PM’s top team
Jeremy Hunt has been tipped to remain as Chancellor, while there has been speculation Suella Braverman could return as Home Secretary
Ms Mordaunt ended up being Mr Sunak’s closest challenger for the Tory leadership and, by convention, she should be offered a job in the new PM’s top team.
The Royal Navy reservist has already held the role of Defence Secretary and is currently the Leader of the House of Commons.
She is likely to be looking for a move up the Cabinet hierarchy, perhaps to one of the ‘great offices of state’.
Could Foreign Secretary suit someone who is a confident media performer?
Ms Mordaunt is a former International Development Secretary and so has experience of managing Britain’s foreign aid budget, which is now under the remit of the Foreign Office.
The most immediate crisis facing Mr Sunak as the incoming PM is the state of the economy in the wake of Ms Truss’s mini-Budget disaster.
Her second Chancellor, Jeremy Hunt, has been working on plans to restore market confidence at a planned fiscal statement on 31st October.
This includes spending cuts and possible tax rises in order to fill a blackhole in the public finances.
With financial markets having reacted well to Mr Hunt’s appointment and his ripping up of Ms Truss’s economic agenda, Mr Sunak will be tempted to keep him in place as Treasury chief.
Mr Hunt last night came out in support of Mr Sunak as PM in a newspaper article.
The final ‘great office of state’ could be filled by Suella Braverman, who is popular on the Tory Right, returning as Home Secretary.
Mrs Braverman also used a newspaper article this weekend to declare her backing for Mr Sunak.
Having been touted as a possible successor to Ms Truss herself, it hinted that Mrs Braverman had extracted promises from Mr Sunak on various home affairs issues.
This included dealing with the Channel migrants crisis by delivering on the Rwanda scheme and possibly reforming the UK’s relationship with the European Convention on Human Rights.
If Mr Sunak is keen to keep the Tory Right on side as he enters No10, why not return Mrs Braverman to the Home Office – a job she only left five days ago.
What are the challenges waiting in Mr Sunak’s in-tray?
Liz Truss’s downfall as PM stemmed from her mini-Budget disaster when her announcement of £45billion in unfunded tax cuts prompted meltdown on the financial markets.
The price of Government borrowing has soared in recent weeks and the Treasury is scrambling to deal with a huge blackhole in the nation’s finances.
Mr Sunak is likely to have to oversee severe public spending cuts and, perhaps, even tax rises in order to balance the books.
He will also have to deal with the continuing cost-of-living crisis, exacerbated by Russia’s invasion of Ukraine, and the harmful impact of runaway inflation.
As soon as she entered No10, Ms Truss promised that energy bills for typical households would be frozen at £2,500 a year until 2024.
But, after her mini-Budget spectacularly unravelled, she was forced to pull apart that promise with the bills bailout now due to end in April.
From that point onwards, financial help is only set to be made available to those most in need.
Will Mr Sunak continue to limit help from April? Or will he give the policy yet another rethink in order to prevent millions of Britons being slapped with huge increases in their bills next Spring?
He will also have to decide whether to increase both pensions and benefits by the rate of inflation, or at a lower level.
Liz Truss’s announcement of £45billion in unfunded tax cuts prompted meltdown on the financial markets
A ‘WINTER OF DISCONTENT’
There are fears that disputes between militant trade unions and employers could worsen further as the cost-of-living crisis continues to bite.
Britons have already been forced to endure a series of nationwide rail strikes in recent months, while Royal Mail workers, barristers, port workers and BT staff have also staged walkouts.
Teachers are set to be balloted on industrial action, with one union having pencilled in strikes from January.
And the Royal College of Nursing is also threatening a walkout in a dispute over pay.
Will Mr Sunak pursue a hardline stance on public sector wages at a time of spending restraint? Or will he not want to risk public anger over weeks’ or months’ worth of disruption?
Britons have already been forced to endure a series of nationwide rail strikes in recent months
The most severe threat to global security remains Russia’s invasion of Ukraine, with Vladimir Putin continuing his brutal assault on his country’s neighbour.
There are fears the Russian President’s barbarism could escalate further if the conflict continues to go against him and his desperation increases.
Mr Sunak will be under pressure to continue Britain’s staunch support for Kyiv, while also keeping in mind how he would react should Mr Putin decide on more drastic action.
He will also face continued demands from Tory MPs to boost defence spending, having refused to commit to a target of raising it to three per cent of GDP.
There are fears Russia’s barbarism could escalate further if the Ukraine conflict continues to go against Vladimir Putin
Relations between Britain and China have deteriorated further after a Hong Kong pro-democracy protester was dragged into the Chinese consulate grounds in Manchester and beaten up.
With an ever-growing number of Sino-sceptics among Tory MPs, how Mr Sunak deals with Beijing is set to be an important test of his premiership.
A Hong Kong pro-democracy protester was dragged into the Chinese consulate grounds in Manchester
Mr Sunak has committed to continuing the Government’s bid to send migrants who enter the UK illegally to Rwanda.
The £120million scheme has still yet to get going, amid a series of legal challenges, despite it being announced back in April.
Former Home Secretary Priti Patel unveiled the plans as a means of clamping down on the number of migrants making perilous journeys to Britain across the Channel.
Mr Sunak will have to decide how he will push forward with the scheme, including possibly overhauling Britain’s relationship with the European Convention on Human Rights.
And, if the scheme continues to stall, the PM will have decide on what further action to take to try to stem the number of migrants arriving in small boats.
The PM will have decide on what further action to take to try to stem the number of migrants arriving from across the Channel in small boats
New elections are due in Northern Ireland if there is no return to a power-sharing government at Stormont before Friday.
The DUP is currently blocking the functioning of Stormont institutions as part of their protest against post-Brexit trading arrangements, known as the Northern Ireland Protocol.
But the party is under pressure to end their boycott of Stormont while resumed talks between the UK Government and the EU over the Protocol continue.
Mr Sunak will hope that increased posititivity around the state of those negotiations with Brussels will soon lead to a breakthrough in the Protocol row.
This should then, hopefully, allow power-sharing to resume in Northern Ireland ahead of the 25th anniversary of the Good Friday Agreement next year.
US President Joe Biden is said to be planning a trip to the UK and Ireland to coincide with the anniversary.
The DUP is currently blocking the functioning of Stormont institutions as part of their protest against post-Brexit trading arrangements, known as the Northern Ireland Protocol
UNITE THE TORY PARTY
After two divisive leadership elections this year and months of bitter infighting between MPs, Mr Sunak faces a huge task in trying to unite the Conservative Party behind him.
If he fails to do so, it is not impossible that the Tories could seek to junk yet another leader before the next general election.
When he abandoned his sensational comeback bid last night, Boris Johnson raised the possibility that he could still chase a return to No10 at another date.
‘I believe I am well placed to deliver a Conservative victory in 2024,’ the ex-PM said.
Mr Sunak faces an immediate question over how to handle Mr Johnson, who obviously believes his career in frontline politics is not over.
The new PM hinted at a possible foreign diplomacy job for Mr Johnson, saying last night: ‘I truly hope he continues to contribute to public life at home and abroad.’
Mr Sunak faces an immediate question over how to handle Boris Johnson, who obviously believes his career in frontline politics is not over.
OVERHAUL LABOUR’S POLL LEAD
The Tories are currently staring at political oblivion on the basis of current opinion polls, which have given Labour huge leads amid the chaos of Ms Truss’s premiership.
Can Mr Sunak turn the Tories’ fortunes around, even as he faces all the numerous challenges listed above?