Just two days ago, the UK's longest reigning monarch received her 15th prime minister in the drawing room of Balmoral Castle, in one of her final acts of duty.
The Queen passed away "peacefully" at the Scottish royal residence on Thursday afternoon, Buckingham Palace said.
Dressed in black outside No 10 on Thursday, Ms Truss addressed the nation: “We are all devastated by the news that we have just heard from Balmoral.
“The death of Her Majesty the Queen is a huge shock to the nation and to the world.”
Ms Truss described her as "a rock on which modern Britain was built" and a "personal inspiration."
“It’s an extraordinary achievement to have presided with such dignity and grace for 70 years. Her life of service stretched beyond most of our living memories," she continued.
“In return she was loved and admired by the people in the United Kingdom and all around the world.
“She has been a personal inspiration to me and to many Britons – her devotion to duty is an example to us all."
“Today the crown passes, as it has done for more than 1,000 years, to our new monarch, to our new head of state, His Majesty King Charles III," she said.
Ms Truss said the country would now support His Majesty, King Charles III.
Following her statement on Downing Street, Ms Truss has spoken to the King, No 10 has confirmed.
“We offer him our loyalty and devotion, just as his mother devoted so much, to so many, for so long," she said in her speech earlier.
“And with the passing of the second Elizabethan age, we usher in a new era in the magnificent history of our great country, exactly as Her Majesty would have wished, by saying the words ‘God save the King’.”
No 10 has confirmed that the prime minister was informed of the Queen's death by the Cabinet secretary at around 4.30pm while working in Downing Street.
On Thursday night, she chaired a meeting with ministers about operational work that must be put in place during the period of mourning.
The House of Commons will sit at noon on Friday for MPs to pay tribute to the Queen in a session due to last until 10pm.
There will also be a rare Saturday sitting, where senior MPs will take the oath to King Charles III, the Commons confirmed.
Buckingham Palace issued a statement earlier on Thursday confirming that the Queen was under medical supervision at Balmoral after doctors became concerned for her health.
It came shortly after Labour leader Sir Keir Starmer rushed out of the Commons Chamber during Ms Truss's speech unveiling her energy plan after he was handed a note. Ms Truss followed a short while later - though it was not immediately clear what news the leaders had received.
Following the news of the Queen's death, the Labour leader paid tribute to her as “a symbol of the best of us” and a "remarkable sovereign".
“We will always treasure Queen Elizabeth II’s life of service and devotion to our nation and the Commonwealth; our longest-serving and greatest monarch," he added.
“Above the clashes of politics, she stood not for what the nation fought over, but what it agreed upon. As Britain changed rapidly around her, this dedication became the still point of our turning world.
“So as our great Elizabethan era comes to an end, we will honour the late Queen’s memory by keeping alive the values of public service she embodied."
A flurry of the Queen's former prime ministers, including Tony Blair and John Major, along with the heads of the devolved nations and Ireland paid emotional tributes to Her Majesty noting it was the "end of an era".
Mr Major described to ITV News the Queen's passing, ending her 70-year reign, as "heartbreaking news" which will be "extremely sad news for millions of people across this country and beyond".
He described Her Majesty as a "remarkable woman".
"The Queen has been in the warp and weft of our lives for 70 years and there are many people in this country who remember nothing but the queen being our monarch," he added.
Former prime minister Boris Johnson said in a lengthy statement shared on Twitter that “this is our country’s saddest day because she had a unique and simple power to make us happy. That is why we loved her.”
He said the Queen had “modernised the constitutional monarchy” and “produced an heir to the throne who will amply do justice to her legacy”.
“Relentless though her diary must have felt, she never once let it show, and to tens of thousands of events – great and small – she brought her smile and her warmth and her gentle humour – and for an unrivalled 70 years she spread that magic around her kingdom," he continued.
“Though our voices may still be choked with sadness, we can say with confidence the words not heard in this country for more than seven decades: God save the King.”
Another of the Queen's former PM's, Gordon Brown, said everywhere he went "Her Majesty was respected, she was admired, she was revered and we will miss her greatly."
He said he was one of the "lucky few" to have met her many times, and said: "She was conscientious, she was considerate, she was caring."She had a great sense of humour. She was endlessly patient - even when talking about the details of a boring Budget."But most of all, what shone through was her complete and utter dedication to the country and the constitution."
"Everywhere I went, Her Majesty was respected, she was admired, she was revered and we will miss her greatly" - Former Prime Minister, Gordon Brown
Former prime minister Sir Tony Blair lamented the UK has "lost not just our monarch but the matriarch of our nation".
"The figure who more than any other brought our country together, kept us in touch with our better nature, personified everything which makes us proud to be British," he added.
Scottish First Minister Nicola Sturgeon said it was a "profoundly sad moment for the UK, the Commonwealth and the world".
"Her life was one of extraordinary dedication and service," she wrote on Twitter. "On behalf of the people of Scotland, I convey my deepest condolences to The King and the Royal Family."
But she added she hoped it would be a "source of comfort" to the Royal Family that "she spent her final days in a place that she loved so much".
Ms Sturgeon said she was "rarely happier than when she was here in Scotland at her beloved Balmoral".
Irish Taoiseach Micheál Martin said the Queen's passing is "the end of an era", adding: "Our world is a poorer place for her passing but a far richer and better place as a result of her long life and enduring contribution."
"The Queen’s reign was one of historic duration, immense consequence and a focus of respect and admiration around the world," he said in a lengthy statement.
"Her dedication to duty and public service were self-evident and her wisdom and experience truly unique."
Wales’ First Minister Mark Drakeford tweeted he was “incredibly sad" to hear of the news.
The UK and world reacts to the death of Her Majesty The Queen at 96
He added: “As our longest reigning monarch, she firmly upheld the values and traditions of the British Monarchy. On behalf of the people of Wales I offer our deepest condolences to Her Majesty’s family during this sad time.”
Sinn Fein vice president Michelle O’Neill said she was personally "grateful for Queen Elizabeth’s significant contribution and determined efforts to advancing peace and reconciliation.”
DUP leader Sir Jeffrey Donaldson said the Queen had been a “steadfast and unshakeable head of state”.
“Her Majesty led by example in Northern Ireland and reached out the hand of friendship to help with the reconciliation process," he added. “We are duty-bound to build on that foundation."
Speaker of the Commons, Lindsay Hoyle, who interrupted the Commons earlier this afternoon when news broke from Buckingham Palace that the Queen was under doctors' supervision, described her as a "constant presence in our lives".
"During her 70 years on the throne – and even before that, as a teenager, reassuring and engaging with children and families disrupted by the Second World War – she has given our lives a sense of equilibrium," he said in a statement.
"While her reign has been marked by dramatic changes in the world, Her Majesty has maintained her unwavering devotion to the UK, the British Overseas’ Territories and the Commonwealth of Nations - and her gentle authority and sound reason have been felt throughout."
He added: "She has been a mother, grandmother and great-grandmother - but she has been our Queen, and we will miss her beyond measure."