After decades as the longest monarch-in-waiting, Charles is now the King of the United Kingdom and 14 other Commonwealth nations following the death of his mother, Queen Elizabeth II.
In line with protocol, the 73-year-old immediately became the 62nd British monarch. A coronation will be held for him to formally take the role of king, making him the oldest person to ever assume the throne.
Clarence House confirmed that Charles will be known as King Charles III.
Charles's wife, Camilla, Duchess of Cornwall, is expected to be given the title of Queen Camilla. “It is my sincere wish that, when that time comes, Camilla will be known as Queen Consort as she continues her own loyal service,” Charles’s late mother said in her accession day letter from February.
Appearing outside of 10 Downing Street, Britain's newly elected prime minister, Liz Truss, confirmed the British monarch as the head of state.
"Today, the crown passes, as it has done for more than 1,000 years to our new monarch, his majesty King Charles III," Truss said.
The Conservative leader paid tribute to the late Queen and called on the support of the British public.
"With the king's family, we mourn the loss of his mother. And as we mourn, we must come together as a people to support him. To help him bear the awesome responsibility that he now carries for us all. We offer him our loyalty and devotion just as his mother devoted so much to so many for so long."
The King has spent his entire life preparing for this moment, his future decided at his birth in 1948 as the eldest child of Queen Elizabeth II and Prince Philip.
He graduated from Cambridge and joined the Royal Navy where he was a flight lieutenant. After completing his navy duty, he founded the "Prince's Trust," a charity focused on improving the lives of disadvantaged young people in the UK and which he has spent the majority of his career working on.
After marrying Lady Diana Spencer in 1981, the pair had two sons, Prince William and Prince Harry, and became a staple of tabloid fodder, before divorcing in 1996.
He married Camilla Parker Bowles, whom he’d been in love with since he was a teenager, in 2005, and has spent recent years mainly focused on his work as an environmental campaigner, advocating for organic farming and climate change mitigation.
Earlier this year, Charles appeared in the House of Lords for the state opening of Parliament to deliver the Queen’s Speech on behalf of his mother, who was unable to attend due to ongoing mobility issues at the time. It was the first time in decades that the Queen had been unable to fulfill this duty marking a new session of Parliament, and it gave citizens a preview of Charles in his new role as the King.