The King was joined by some of his closest friends and family members for his private birthday dinner at Clarence House.
Pictured leaving Charles's London residence on Tuesday evening were the Prince and Princess of Wales, Princess Beatrice and Zara and Mike Tindall, who all put on a glamorous display.
Kate looked stunning, donning a glittering emerald Needle and Thread gown, while William looked suave in a tuxedo.
The King's niece Zara looked gorgeous in black LBD with glittering pearl accents, which she wore for the relaunch of Rebecca Vallance in Harrods earlier in the day.
She held hands with husband and former rugby star Mike as they left the royal residence.
Meanwhile, Princess Beatrice appeared to be wearing a velvet number as she waved from behind the wheel of her car. She did not appear to be joined by husband, Edoardo Mapelli Mozzi, who turns 40 later this week.
Beatrice's mother, Sarah, Duchess of York did not appear to be among the guests, but she shared an image of King Charles on Instagram to wish him Happy Birthday.
It's unlikely that Charles's siblings attended the event, given the fact that they had evening engagements elsewhere.
The Duke of Edinburgh was at a reception for the Commonwealth Games Federation at Swissȏtel the Stamford in Singapore, while his wife, the Duchess of Edinburgh attended a dinner at Rochester Cathedral as patron of the Rochester Cathedral Trust.
The Princess Royal carried out engagements in Glasgow on Tuesday and was among the guests at Interfaith Glasgow’s Scottish Interfaith Week Forum at Glasgow City Chambers.
The King was joined by his maternal cousins, David Armstrong-Jones, 2nd Earl of Snowdon and Lady Sarah Chatto - the children of the late Princess Margaret and Antony Armstrong-Jones.
Charles has remained close to his cousins over the years and they have attended major royal events, including his coronation at Westminster Abbey in May.
The King marked his milestone 75th birthday by launching an initiative to support charities feeding the nation with unwanted food – and surprised a Big Issue seller with a generous donation.
Charles visited a food distribution hub in Oxfordshire to kick-start his Coronation Food Project with the Queen and found himself twice serenaded with verses of Happy Birthday.
The King was photographed by renowned celebrity photographer Rankin for the latest Big Issue edition, where he penned an op-ed about food waste. He also handed Kelvin £10 for the magazine costing £4, at the end of the event.
The 61-year-old, who has been homeless for periods of his life since a teenager, joked afterwards: "He gave me cash, that does prove something – he does carry money."
Kelvin, whose pitch is outside Somerset House in central London, added: "He asked if I was in accommodation and about selling the Big Issue. I said it's got me through the bad times and it has a positive social message."
He said about the Coronation Food Project: "I think it goes to show he cares, he’s reaching out to the general public."
Charles and Camilla were given a tour of the South Oxfordshire Food and Education Alliance, part of FareShare, a national network of charitable food redistributors.
The King also hosted a reception for nurses and midwives at Buckingham Palace as part of the NHS' 75th anniversary celebrations.
And on Monday, the King joined fellow 75-year-olds for a tea party at his Gloucestershire home, Highgrove, where he was presented with a three-tiered Victoria Sponge cake.
Watch his reaction in the video below…
The Prince and Princess of Wales also shared a heartfelt public tribute to Charles on their social media platforms.
The Duke of Sussex reportedly called his father on his special day and according to The Telegraph, Charles received a recorded video of his grandchildren Prince Archie and Princess Lilibet singing Happy Birthday.
Relations between Prince Harry and King Charles have been strained in recent years, since the Duke and Duchess stepped down as senior royals in 2020.
LISTEN: All about the royal family's love stories on the latest episode of HELLO!'s A Right Royal Podcast