In front of a congregation including about 100 world leaders and a television audience of millions, the Archbishop of Canterbury, the spiritual leader of the Anglican Church, slowly placed the 360-year-old St Edward's Crown on Charles' head as he sat upon a 14th-century throne in Westminster Abbey.
During a historic and solemn two-hour service, which dates back to the time of King William the Conqueror in 1066, Charles' second wife Camilla was also crowned Queen.
A huge military procession followed, gun salutes were fired, thousands of soldiers roared three cheers and there was a scaled-down flypast by military aircraft as the newly-crowned King and Queen waved from the balcony of Buckingham Palace to cheering crowds who gathered on The Mall boulevard.
While rooted in history, the ceremony - televised for only the second time - was also an attempt to present a forward-looking institution and to reflect a more diverse country with all its religions.
"No other country could put on such a dazzling display - the processions, the pageantry, the ceremonies, and street parties," Prime Minister Rishi Sunak said.
Charles, 74, automatically succeeded his mother as king on her death last September.
The coronation is not essential but regarded as a means to legitimise the new sovereign in a public way.
Saturday's event was on a smaller scale than that staged for Queen Elizabeth in 1953 but still sought to be spectacular.
There was an array of historical regalia from golden orbs and bejewelled swords to a sceptre holding the world's largest colourless cut diamond.
After the service, Charles and Camilla, 75, departed in the four-tonne Gold State Coach to ride to Buckingham Palace in a one-mile procession of 4000 military personnel from 39 countries.
Tens of thousands of people ignored pouring rain to mass on the streets to watch what some saw as a moment of history.
Inside the abbey, which was bedecked with flowers and flags, politicians and dignitaries from around the world such as US first lady Jill Biden took their seats alongside charity workers and celebrities, including actors Emma Thompson, Maggie Smith, Judi Dench, Australian singer Nick Cave and US singer Katy Perry.
Charles looked solemn as he swore oaths to govern justly and uphold the Church of England - of which he is the titular head.
He was then hidden from watching eyes by a screen for the most sacred part of the ceremony when he was anointed on his hands, head and breast by Archbishop of Canterbury Justin Welby with holy oil consecrated in Jerusalem.
After being presented with symbolic regalia, Welby placed the St Edward's Crown on his head and the congregation cried out "God save the King".
Gun salutes were fired at the Tower of London and across the capital, the UK, in Gibraltar, Bermuda and on ships at sea.
His eldest son and heir Prince William, 40, knelt before his father to pledge his loyalty as his "liegeman of life and limb," both moments greeted by cheers from crowds outside.
There was no formal role for either Charles' younger son Prince Harry, after his high-profile falling out with his family, or his brother Prince Andrew, who was forced to quit royal duties because of his friendship with late US financier Jeffrey Epstein, a convicted sex offender.
They sat in the third row behind working members of the royal family and neither appeared on the palace balcony.
Not everyone who came to watch was there to cheer Charles, with hundreds of republicans booing and waving banners reading "Not My King".
The Republic campaign group said its leader had been arrested along with five other protesters.
