As the prime minister with the shortest time in office officially left Downing Street while the first British Asian PM took her place, this was a momentous day in politics.
On top of that, there was a dramatic reshuffle, as Sunak seeks to separate himself from the premierships of the last two prime ministers while bringing back some surprise faces to cabinet.
Here’s everything you need to know about what happened.
1. Liz Truss says farewell
The prime minister with the shortest tenure in UK history said goodbye to Downing Street at 10.15am.
She urged her successor to be “bold” and stick to her tax-cutting agenda, despite the economic chaos it caused and the widespread criticism it evoked.
She offered no apology in her speech and urged Sunak to focus on “growth” by cutting taxes and reducing public spending, maintaining: “I am more convinced than ever we need to be bold and confront the challenges that we face.”
Truss said: “We simply cannot afford to be a low growth country where the government takes up an increasing share of our national wealth and where there are huge divides between different parts of our country.
“We need to take advantage of our Brexit freedoms to do things differently.”
The vast majority of her cabinet were later replaced or reshuffled after less than two months in the job.
Her deputy prime minister and health secretary Therese Coffey – now the environment secretary – captured the moment with a selfie on Downing Street.
2. Sunak meets the King
Shortly after Truss met with the King to resign, her successor filed into Buckingham Palace so he could officially be confirmed as the next prime minister.
This was a historic moment, marking the beginning of the first UK premiership led by a Hindu and British Asian. It’s also only the second time a prime minister from an ethnic minority has taken the country’s reins, too.
At 42, Sunak is also the youngest prime minister in more than 200 years.
3. Sunak’s welcome speech
Sunak started his premiership with a bang, by immediately addressing the “mistakes” of his predecessor in a speech before midday.
He said: “Some mistakes were made – not born of ill will, or bad intentions. But mistakes nonetheless.
“And I have been elected as leader of my party and your prime minister in part to fix them.
“And that work begins immediately.”
Sunak also used the opportunity to address former prime minister Boris Johnson’s claim that he was “uniquely” positioned to lead the Tories – despite withdrawing from the race.
4. An exodus of ministers
Business secretary Jacob Rees-Mogg led a wave of resignations from the cabinet. A prominent ally to both Johnson and Truss, he has not spoken favourably of Sunak in the past, declaring him a “socialist” during the Tory leadership election in the summer. He resigned with a handwritten letter dated ‘St Crispin’s Day’.
Chair of the Conservative Party Jake Berry, levelling up secretary Simon Clarke, justice secretary Brandon Lewis, education secretary Kit Malthouse and work and pensions secretary Chloe Smith all followed him out the door.
Robert Buckland also stood down as Welsh secretary as did Wendy Morton, the chief whip during Truss’s time in office, environment secretary Ranil Jayawardena and the minister for development Vicky Ford.
Alok Sharma was removed from the cabinet too, but remains as Cop26 president, meaning he will negotiate on the UK’s behalf at Cop27, but won’t attend cabinet or be known as a minister.
5. A new cabinet emerges
Sunak is under pressure to unite the fractured Conservative Party through his leadership by appointing MPs from all its factions to government positions.
And he’s certainly caused a stir with his new appointments.
Sunak’s loyal ally, former deputy PM and former justice secretary Dominic Raab was also rewarded, and handed his old two titles again.
Michael Gove returns to cabinet as levelling up secretary, just two months after he said he was quitting frontline politics.
A few more old faces returned to cabinet amid the reshuffle.
Suella Braverman, who resigned as home secretary a week ago over breaching security rules, returns to that same job.
Former Welsh secretary Simon Hart has become the new chief whip while James Cleverly kept his job as the foreign secretary and Ben Wallace kept his as defence secretary.
Former education secretary Gavin Williamson also returned to cabinet as a minister without portfolio. He was the chief whip under Theresa May.
Penny Mordaunt, Sunak’s rival in the leadership contest, was also appointed the leader of the Commons, while Truss’s close ally Therese Coffey became environment secretary.