The former Foreign Secretary defeated her opponent Rishi Sunak, and won 57% of the party’s vote to become the next Conservative leader.
It was an eventful day for British politics, with build-up and reaction - and even resignations - coming in thick and fast following the announcement.
It may have been hard to follow at times, so here is your complete breakdown for the key points in the day.
What happened as Liz Truss was announced new Prime Minister?
MPs began arriving at the Queen Elizabeth II Centre in Westminster around an hour and a half before Sir Graham Brady delivered the final vote tally.
A large media scrum welcomed the Tory MPs to the venue.
Former candidates including Penny Mourdant, who was the favourite to win in the early stages of the contest, made their way into the centre.
A man is arrested outside the Queen Elizabeth II Centre after attempting to stage a protest for Animal Rebellion.
Four other protesters sat in the road to the venue to block MPs getting into the building. As police officers attempted to speak to them, the group chanted “Protect our planet, respect our future”, while holding placards which read “plant-based future”.
Despite the minor disruption from protesters outside the venue, the results are announced as planned.
Sir Graham Brady, chairman of the 1922 committee which oversees inner-party elections, arrives on stage to announce the results after a seven-week long contest.
Sir Graham confirmed that Liz Truss had won the race, gaining 81,326 votes compared to Rishi Sunak’s 60,399.
This equates to a 57% share of the vote.
Liz Truss takes to the stage to give her victory speech.
In her address, the newly-elected prime minister and party leader said: “Thank you for putting your faith in me to lead our great Conservative Party, the greatest political party on earth.
“I know that our beliefs resonate with the British people: our beliefs in freedom, in the ability to control your own life, in low taxes, in personal responsibility.
“I know that’s why people voted for us in such numbers in 2019 and as your party leader I intend to deliver what we promised those voters right across our great country.”
Ms Truss commended her opponent Rishi Sunak, while also paying tribute to her predecessor Boris Johnson, saying: “Boris, you got Brexit done, you crushed Jeremy Corbyn, you rolled out the vaccine and you stood up to Vladimir Putin. You were admired from Kyiv to Carlisle.”
In the closing statements of her speech, the former Foreign Secretary promised that as leader of the party and the country “we will deliver, we will deliver and we will deliver.”
Despite the audience appearing to be unsure whether to clap or not at the end of her tribute to Boris Johnson, come the end of her speech overall, she received a standing ovation.
Reaction from the UK political-sphere began rolling in shortly after Ms Truss’s victory was announced.
Liberal Democrats leader Sir Ed Davey called for an immediate general election following the leadership vote, while Labour leader Sir Keir Starmer criticised her lack of commitment toward tackling the cost of living crisis.
He said: “We’ve heard far more from the latest prime minister about cuts to corporation tax over the summer than we have about the cost-of-living crisis, the single most important thing that’s bearing down on so many millions of households.
“That shows not only that she’s out of touch, but she’s not on the side of working people. So she needs to deal with the cost-of-living crisis, she needs to deal with the fact the NHS is on its knees, and she needs to deal with the collapse of law and order.”
However, former Prime Minister Theresa May was in more an congraulatory mood, tweeting: “We Conservatives must now work together to address the challenges facing our country.
“Tackling the cost of living, delivering for those in need & managing the public finances responsibly. I look forward to supporting the Government in that task.”
One of the most controversial moments of Ms Truss’s leadership campaign came when she branded Scottish First Minister Nicola Sturgeon an “attention seeker” when asked about how she would handle the possibility of a second Scottish independence referendum.
However, in a tweet acknowledging Ms Truss’s victory, Ms Sturgeon said that she would “seek to build a good working relationship” with the new Prime Minister.
She said: “Congratulations to Liz Truss. Our political differences are deep, but I will seek to build a good working relationship with her as I did with last three [prime ministers].
“She must now freeze energy bills for people & businesses, deliver more cash support, and increase funding for public services.”
Rishi Sunak acknowledged his opponents win with a Twitter message shortly after the result event was over.
The former Cahncellor said: “Thank you to everyone who voted for me in this campaign.
“I’ve said throughout that the Conservatives are one family.
“It’s right we now unite behind the new PM, Liz Truss, as she steers the country through difficult times.”
European Commission president Ursula von der Leyen sent Ms Truss a message of conrgatulations, while also emphasising the importance of creating a “constructive relationship” between the UK and the European Union.
She tweeted: “The EU and the UK are partners. We face many challenges together, from climate change to Russia’s invasion of Ukraine.
“I look forward to a constructive relationship, in full respect of our agreements.”
Ms Truss arrived at the Conservative Campaign Headquarters in Westminster alongside Work and Pensions Secretary Therese Coffey, who is a strong ally of the new Prime Minister.
While inside the venue, Ms Coffey told reporters that Ms Truss will announce appointments to her cabinet on Tuesday, after she is formally sworn in by the Queen at Balmoral Castle.
After a day of jubilation for certain members of the Tory party, Ben Elliot, an ally of Boris Johnson, announced his resignation as the party’s co-chiarman.
Sources close to the former co-chairman say that Mr Elliot’s decision to resign was not influenced by the victory of Ms Truss.
Conservative Party chief executive Darren Mott also added: “The whole Conservative Party wants to thank Ben Elliot for his tireless service over the past three years.
“Without his incredible efforts, the 2019 landslide would not have been possible. We wish him all the best in his future endeavours.”
Volodymyr Zelensky sent one last tribute to his close ally Boris Johnson ahead of his departure.
Mr Zelesnky said: “Had a summing up conversation with Boris Johnson (@BorisJohnson) in his current capacity.
“On behalf of all (Ukrainian) people, I thanked him for his personal bravery, principles & a major contribution to countering RF’s aggression.
“I look forward to cooperation with a great friend of (Ukraine) in a new status.”
Home Secretary Priti Patel announces she is resigning from her cabinet role ahead of Liz Truss’s premiership.
Ms Patel, a long-standing ally of Boris Johnson, commended her and her Prime Minister’s record in government, particularly their policy and stance on immigration and crime.
Although Ms Patel never gave a specific reason for her resignation, it comes as Liz Truss looks to assemble a new cabinet under her leadership.
Therese Coffey is highly tipped to take on the role of Home Secretary in Ms Truss’s government, while former Northern Ireland Secretary Brandon Lewis is also tipped for an appointment.