Charles welcomed South Africa's President Cyril Ramaphosa earlier today in another milestone for the new monarch.
The last state visit by a world leader was in 2019 with former US President Donald Trump and was held in the gardens of Buckingham Palace.
Prince William and Kate Middleton also arrived wearing their evening best. Kate was seen wearing the Lovers Knot tiara, which used to be a favourite of Princess Diana.
Other members of the royal family that have arrived include the Earl and Countess of Wessex, the Duke and Duchess of Gloucester and the Duke of Kent.
Mr Ramaphosa was warmly greeted by the monarch and the Queen Consort Camilla on a chilly Horse Guards Parade as he began a two-day state visit to the UK.
The Prince and Princess of Wales travelled to Mr Ramaphosa's luxury hotel in nearby central London and accompanied him to Horse Guards Parade for the start of the ceremonial welcome.
Prime Minister Rishi Sunak and senior members of the Cabinet were also there for the ceremonial reception.
Buckingham Palace is hosting a state dinner tonight, but due to reservicing work, the South African leader will not be staying there.
Work for the South African state visit began during the late Queen's reign and the King was said to have been delighted to continue with the plans.
The prince and princess also are also in attendance tonight at the white-tie banquet, with Kate and Camilla in tiaras.
Both the King and the president are expected to make speeches.
The visit follows the traditional format adopted during the reign of the late Queen.
Tonight's menu however differs slightly from the Queen's traditional Balmoral lamb or venison.
His Majesty is treating Mr Ramaphosa to a luxury menu of stuffed Windsor pheasant and grilled brill with wild mushrooms tonight.
The lavish menu offers two mains of grilled brill and Windsor Pheasant.
It differs slightly from the menu of Balmoral venison or lamb from when Queen Elizabeth II hosted.
Some of the sides included a selection of Chantenay carrots, kale with roasted butternut squash, braised fondant potatoes and salad.
And for dessert, guests will enjoy an iced vanilla parfait with caramelised apples or coffee and bite-sized confectionery were the options for dessert.
After the ceremony today, the King hosted the president at a private lunch at the Palace and then invited him to view an exhibition in the Picture Gallery of items from the Royal Collection relating to South Africa.
When Mr Ramaphosa picked up a photograph of the Queen with former president Mr Mandela during a 1996 Buckingham Palace state banquet, he said: "This lovely picture," and agreed when the King replied: "You were lucky to have known both."
As they came across a photograph of Charles with the Spice Girls band in South Africa in 1997, the president said: "There you are," with Charles adding with a smile: "There they are."
The Prince of Wales later spotted the photo, saying with a grin to his accompanying guests: "My father in South Africa - the Spice Girls."
Tomorrow, the Earl of Wessex will escort the South African leader to the Royal Botanic Gardens in Kew.
It comes after the King asked Parliament to add his youngest brother Edward - and sister the Princess Royal - as extra Counsellors of State so he can deputise for the monarch if need be.
Mr Ramaphosa will also visit Downing Street to meet Prime Minister Rishi Sunak, before returning to the Palace to bid farewell to the King.
Mr Sunak said: "South Africa is already the UK's biggest trading partner on the continent, and we have ambitious plans to turbocharge infrastructure investment and economic growth together.
"I look forward to welcoming President Ramaphosa to London this week to discuss how we can deepen the partnership between our two great nations and capitalise on shared opportunities, from trade and tourism and security and defence."