The 75-year-old monarch, who has been battling an undisclosed form of cancer since February, was seen beaming as he greeted guests at the evening event in St Paul's Cathedral, which followed the British government's International Investment Summit.
Charles' appearance comes as he prepares for an 11-day tour of Australia and Samoa starting on Friday.
He is reportedly taking two doctors and a supply of his blood on the trip, which will be his first to a Commonwealth nation since becoming King in May.
It is understood he is pausing his chemotherapy treatment until after the tour.
The monarch, 75, is preparing for an 11-day tour of Australia and Samoa. (AP PHOTO)
At the London event Charles was decked out in a navy suit with a patterned blue-and-white tie, set off by a matching handkerchief.
He smiled as he chatted with international business leaders, along with other notable guests including Prime Minister Keir Starmer and Andrew Lloyd Webber.
A dinner for the event was prepared by three Michelin-starred chef Clare Smyth while Elton John was set to perform at the gathering.
Sir Elton has years-long links to the royal family, famously performing at Princess Diana's funeral.
The singer also attended the late Queen Elizabeth's Diamond Jubilee concert in 2012, appearing onstage next to the monarch and future King and Queen Charles and Camilla.
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