The former chancellor won the race when rival Penny Mordaunt failed to gain the backing of enough MPs and after Boris Johnson ended his comeback attempt.
On Tuesday, Ms Truss will take a short car ride from Downing Street to Buckingham Palace for an audience with King Charles, in which she will formally tender her resignation.
Once she has left, it is Mr Sunak’s turn to see the king at the palace. He will be appointed the country’s next prime minister and asked to form an administration.
Mr Sunak will then head back to Downing Street to address the nation for the first time as prime minister at about 11.35am.
It is customary for the new prime minister to be greeted by the Cabinet Secretary at the door of No 10 and be clapped in by staff before heading into the Cabinet Room to receive security and intelligence briefings from civil servants.
Mr Sunak will be handed the nuclear codes and write “letters of last resort” to commanders of submarines carrying Trident nuclear missiles, with orders on what to do if the government has been wiped out in a nuclear attack.
He could also start receiving calls from world leaders wishing him well in the new job.
Who is tipped to make Mr Sunak's Cabinet?
Mr Sunak is likely to use the rest of Tuesday putting together his new Cabinet as he seeks to fix a fractured party.
To many, Liz Truss made a key error in appointing loyalists to top roles in her Cabinet.
An early and consistent backer of Mr Sunak, Dominic Raab, is likely to be tipped for a return to Cabinet.
As justice secretary he had long pursued a plan to overhaul the Human Rights Act – only to see it shelved by Ms Truss after she entered office.
The former deputy prime minister, whose marginal seat could be under pressure at the next general election, will hope to see his loyalty rewarded with a role that allows him to deal with his own priorities.
The Leader of the House of Commons, Penny Mordaunt, pulled out of the race to be the next Tory leader at the last moment, allowing Mr Sunak to be crowned as Ms Truss’s replacement.
She said that Mr Sunak had her “full support”.
Ms Mordaunt, who still has a sizeable number of enthusiastic backers in the parliamentary party, is tipped for some kind of promotion, with some speculating that she could replace James Cleverly as foreign secretary.
Rishi Sunak through the years - in pictures
The current Chancellor Jeremy Hunt was brought in to steady Ms Truss’s ailing government.
By ripping up her economic vision, Mr Hunt effectively took charge of the economic agenda and in doing so brought some reassurance to the markets.
In the name of stability, Mr Hunt could be in line to stay in the Treasury. It helps that he backed Mr Sunak.
He has also been working towards a highly anticipated fiscal statement on October 31, but it remains to be seen if he will be the person who delivers it in the Commons.
Current Deputy Prime Minister Therese Coffey is a close friend and ally of Ms Truss.
On Monday, sheMs Coffey said that the party must get behind Mr Sunak, but it still appears unlikely that the next prime minister will want to keep her in the key role of Health Secretary.
Briefly home secretary, Suella Braverman’s backing for Mr Sunak was a key sign that the right wing of the Conservative Party was turning towards the former chancellor.
She left with a scathing assessment on the “tumultuous” Truss administration and might be hoping for a swift return to the Home Office in the coming days.
Mel Stride, a long-time backer of Mr Sunak and the Commons treasury committee chairman, was a thorn in the side of Ms Truss and Kwasi Kwarteng as the pair pushed ahead with the mini-budget without any forecast from the Office for Budget Responsibility.
Mr Stride could now be in line for a key role in the Sunak government, possibly in the Treasury or elsewhere.
Another opponent of Ms Truss, Grant Shapps was drafted in as home secretary to replace Ms Braverman. The loyal Sunak supporter and former transport secretary can probably expect to remain in the Cabinet.
Business Secretary Jacob Rees-Mogg is one of the most vocally loyal backers of Boris Johnson in the party.
Having once derided Mr Sunak as a “much-lamented socialist chancellor”, he said on Monday he will “support his leadership” and urged “now is the time for party unity”.
If Mr Sunak wants to show that he can unite the party, Mr Rees-Mogg could be a key figure to keep in government.
Nonetheless, the fracking enthusiast may find himself moved from the Department for Business, Energy and Industrial Strategy.
Rishi Sunak - in pictures
The government Chief Whip and Truss loyalist, Wendy Morton, is highly likely to be moved on.
Ms Morton's efforts in maintaining party discipline were heavily criticised in recent days, particularly over Labour’s motion on fracking.
A vocal backer of an increase in defence spending, Mr Wallace could clash with Mr Sunak and Mr Hunt amid talk of widespread spending cuts.
He also admitted that he was “leaning towards” Mr Johnson as Ms Truss’s replacement, but he could stay on in the name of stability.
Matt Hancock was health secretary during the coronavirus pandemic but he quit in June 2021 after leaked CCTV footage showed him breaching social distancing rules by kissing aide Gina Coladangelo in his office.
He backed Mr Sunak in this latest contest, declaring on Friday that the former chancellor was “the best person to lead our country”.
But on Monday, footage of Mr Sunak greeting supporters appeared to show him snub Mr Hancock who was smiling and clapping enthusiastically as the new leader hugged and shook hands with others.
Mr Johnson pulled out of the race to replace Ms Truss, killing the hopes of his supporters for a glorious return to Downing Street.
In his statement pulling out of the race, he said: “I believe I have much to offer but I am afraid that this is simply not the right time,” he said after pulling out.
Mr Sunak could extend an olive branch to Mr Johnson by offering him a role, but it is an open question whether he would accept.
Rishi Sunak delivers speech as incoming UK prime minister - video
On Wednesday, Mr Sunak is set to face Labour leader Sir Keir Starmer during his first Prime Minister’s Questions at noon.
He is likely to continue his reshuffle and receive more calls from international leaders as he starts working through his in-tray.
Mr Sunak faces a daunting set of challenges, exacerbated by the chaotic legacy left by Ms Truss, including turbulent financial markets and industrial unrest.
Monday, October 31, was a date set by Ms Truss and Mr Hunt for the highly anticipated fiscal statement setting out how the government intends to get the public finances back on track.
It remains to be seen whether Mr Sunak will stick to this timetable.
Mr Hunt has warned of “eye-wateringly difficult” decisions ahead on tax and spending. Mr Sunak, who has emphasised the need for financial stability, may take a broadly similar approach.
In November, Mr Sunak is expected to make his first international trips as prime minister, travelling to the Cop27 climate summit in Egypt in early November and to the G20 in Bali a week later.
Rishi Sunak’s rise is ‘our Barack Obama moment’
Rishi Sunak becoming prime minister is “our Barack Obama moment”, said the president of a Hindu temple established by the new Tory leader's grandfather.
Mr Sunak is the first practising Hindu to become British prime minister and his success has been hailed as a “historic moment” that shows the highest office “can be open to those of all faiths and ethnic backgrounds”.
The Vedic Society Hindu Temple in Southampton was established by Mr Sunak's grandfather Ramdas Sunak, in 1971, and his father, Yash, was a trustee during the 1980s.
Mr Sunak takes his House of Commons oath on the Bhagavad Gita, a sacred Sanskrit text, and he regularly visits the Southampton temple.
Rishi Sunak declared next leader of UK Conservative Party - video
He was born to parents of Punjabi descent. His grandparents were born in India and emigrated to the UK from East Africa in the 1960s.
“He comes regularly to the temple, even now,” temple president Sanjay Chandarana said.
“He did that in July, just a couple of days before Rishi resigned as chancellor, they offered a family prayer here and he came for the lunch.
“I am sure as soon as he becomes prime minister he will definitely come to the temple for prayer. I know that is going to happen.”
He added: “It's a proud moment, the temple is buzzing right now, a lot of people are showing their own pictures with him. When he was here before, he posed for pictures with every single person who was in temple, 300 people.
“It's like for the UK, it's the Barack Obama moment, where a non-white person becomes prime minister for the first time, also a person from Indian origin and Hindu which is another dimension and everyone is very proud.”
Mr Chandarana said that Mr Sunak's appointment as PM showed that integration was working in the UK.
“It will unite the country, because he practises Hindu religion religiously and one of the key values we have is the whole world is our family and we believe in unity in that respect.
“The biggest challenge is the economic challenge and the political uncertainty, that is something he will have to address.”
Mr Chandarana said that Mr Sunak's win coincided with Diwali, making it an auspicious moment and an extra reason for his temple to celebrate.
“We increased our firework time from 15 minutes to 20 minutes. Also when we learnt he was going to be prime minister we offered a special prayer and he will feature in my Diwali speech,” he said.
“It's going to be another festival within a festival, the icing on the cake.”
Sunder Katwala, director of British Future, said: “Rishi Sunak becoming the first British Indian prime minister is a historic moment. This simply would not have been possible even a decade or two ago.
“It shows that public service in the highest office in Britain can be open to those of all faiths and ethnic backgrounds.
“This will be a source of pride to many British Asians — including many who do not share Rishi Sunak's Conservative politics.”
He added: “Most people in Britain now rightly say the ethnicity and faith of the prime minister should not matter.
“They will judge Sunak on whether he can get a grip on the chaos in Westminster, sort out the public finances, and restore integrity to politics.
“But we should not underestimate this important social change.”
He pointed out that when Mr Sunak was born, there had been no Asian or black MPs in the postwar era.
He said Mr Sunak's rise “does not make Britain a perfect meritocracy”, but while there is “more to do”, it is a “hopeful sign of progress against the prejudices of the past”.
The Hindu Forum of Britain tweeted that Mr Sunak had written to the group sharing his best wishes with those celebrating Diwali, and said they were wishing him in return “every success on this auspicious day”.