The first dominoes started to fall within hours of the foreign secretary officially defeating rival Rishi Sunak in the premiership race, with Priti Patel quitting as home secretary on Monday night as Ms Truss looked to assemble her new-look Cabinet.
Ms Truss had earlier defeated the former chancellor by 81,326 votes to 60,399 – earning 57 per cent of the eligible vote from Tory members in a margin that was less decisive than had been tipped in some quarters.
In a maiden speech that focused mainly on Boris Johnson – the outgoing leader she replaces in Downing Street from Tuesday – Ms Truss said her beliefs of freedom, low taxes and personal responsibility “resonate with the British people”.
She said: “During this leadership campaign, I campaigned as a Conservative and I will govern as a Conservative.
“We need to show that we will deliver over the next two years. I will deliver a bold plan to cut taxes and grow our economy.
“I will deliver on the energy crisis, dealing with people’s energy bills, but also dealing with the long-term issues we have on energy supply.”
Appearing to rule out an imminent general election, Ms Truss also promised Tory members “we will deliver a great victory for the Conservative Party in 2024”.
Ms Truss will fly to Balmoral to meet the Queen for the formal handover of power on Tuesday after Mr Johnson makes the same journey to resign in an official ceremony involving the monarch.
The biggest cheer of Ms Truss’s speech was reserved for praise of Mr Johnson, with loud applause after the Prime Minister elect called him her “friend”.
She added: “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.”
That she did not go as far as Scotland was not unnoticed by those in the room.
Ms Truss had not enjoyed the support of the majority of MPs during the parliamentary stage of the contest, having only overtaken Mr Sunak in the contest’s final weeks.
Her start as Prime Minister was dealt an early blow by the margin of victory, which was much closer than expected and narrower than the past three Tory leadership contests that went to a vote.
As well as dealing with a daunting set of economic challenges, along with domestic and international political problems including the war in Ukraine, Ms Truss will have to reunite a Tory party that has spent weeks indulging in blue-on-blue infighting.
She said it had been a “hard-fought campaign” that showed “the depth and breadth of talent” in the party.
In the wake of Ms Truss’s speech at the Queen Elizabeth II Centre in Westminster, Ms Patel said it was her “choice” to continue her public service from the backbenches in formally announcing her resignation by social media.
While she pledged her support for the new leader, Ms Patel said it was “vital” that she continued to support the policies she had pursued to tackle illegal immigration, including the deportation of asylum seekers to Rwanda.
“It has been the honour of my life to serve as home secretary for the last three years,” she tweeted.
“I am proud of our work to back the police, reform our immigration system and protect our country.”
Nicola Sturgeon warned there would be “no time to waste” when it comes to tackling the cost-of-living crisis.
The First Minister pledged to build a “positive, constructive working relationship” with the new Conservative leader – despite Ms Truss branding Ms Sturgeon an “attention seeker” in the contest to succeed Mr Johnson, and saying she would “ignore” the SNP leader.
Ms Sturgeon said: “I wish Liz Truss well as Prime Minister. I congratulate her on her appointment. It’s a big responsibility.
“I know how difficult leadership is in the best of times, and these are not the best of times.”
Ms Sturgeon said “despite our political differences” she would look to work with the Conservative leader as she added: “She’s got big decisions that she needs to take, not in the weeks ahead, but in the hours ahead.”
The First Minister urged Ms Truss to “freeze energy bills and not allow the October increase to go ahead”, as well as calling on her to deliver some respite for businesses from mounting energy costs.
She said Ms Truss needed to “give more cash support to people who are already struggling to pay their heating bills or to feed their children, and to deliver more funding for devolved governments to protect public services and public sector workers”.
Ms Sturgeon added: “These are big responsibilities and there is no time to waste. Not just Liz Truss’s premiership, but the future of the UK and the lives of people in every part of the UK will be affected by the decisions she takes in the days to come.”
Mr Sunak said the Tories were “one family” and “it’s right we now unite behind the new PM, Liz Truss, as she steers the country through difficult times”.
Labour leader Sir Keir Starmer 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.”
European Commission president Ursula von der Leyen and vice-president Maros Sefcovic both congratulated the incoming Prime Minister, but made calls for a better working relationship.
Mr Sefcovic, who is the EU’s lead negotiator on the protocol impasse, tweeted: “A positive relationship is of great strategic importance. I stand ready to work intensively and constructively with my new UK interlocutor to foster such a partnership, in full respect of our agreements.”
Ms von der Leyen added: “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.”
Elsewhere business groups called for “decisive action” to avoid company failures as they tackle rocketing energy bills.
Stephen Phipson, chief executive officer of manufacturers’ body Make UK, said: “We congratulate Liz Truss on her appointment and look forward to discussing the critical importance of manufacturing, a sector that delivers millions of well-paid, highly-skilled jobs across the whole of the UK.
“But industry at this time needs decisive action from the new Government to help it through a prolonged period of unprecedented hikes in energy bills, rising cost of raw materials and critical labour shortages in order to keep Britain’s place as a leader of innovation on the world stage.”
Union leaders were scathing about how long it has taken to elect a new prime minister and said the top priority must be tackling the cost-of-living crisis.
Unison general secretary Christina McAnea said: “Leaving the country rudderless all summer at a time of great emergency has been nothing short of a national disgrace.
“The Government has got to get back to governing immediately. Liz Truss must do what should’ve happened months ago and deliver help to the millions unable to cope with their crushing bills.
“Many family finances may never recover without an urgent assistance plan. Tackling the cost-of-living crisis must be the Prime Minister’s number one priority, not wasting precious time attacking unions for trying to help working people through the pain.
“Hard on the heels of an energy lifeline must be an above-inflation wage rise for the public services currently haemorrhaging staff to better-paying parts of the economy. If there’s no-one left to run the hospitals, schools, town halls, police stations and care homes communities rely on, we’ll all be done for.
“Cutting taxes only assists the better-off. It won’t help the hospital porters, teaching assistants, care staff or other low-paid workers one bit.”