Charles spoke warmly with many of the attendees, including asking Olympic swimmers how long they could hold their breath, as he joined other royal family members at the event on Wednesday.
It comes after Charles returned to public-facing duties last week following his cancer diagnosis and marked the first anniversary of the King and Queen's coronation.
About 8000 people were in attendance for The Sovereign's Garden Party on a sunny afternoon at Buckingham Palace.
Charles, wearing a grey suit and light blue waistcoat and tie, smiled to the gathered crowd as he entered the garden from the palace's Carnarvon Room.
He emerged alongside Camilla who wore a white silk dress by Fiona Clare and a hat by Philip Treacy, as well as a heart-shaped diamond brooch that belonged to the late Queen Elizabeth II.
The pair were also joined by the Princess Royal, the Duke and Duchess of Edinburgh and the Duke and Duchess of Gloucester.
All the family members stood at the garden steps to observe the national anthem, before Charles shook his top hat with joy when the crowd cheered loudly as the song ended.
Charles and Camilla then greeted sections of the crowd and groups of attendees separately.
One attendee who spoke with Charles said "God bless you" while another remarked: "Good to see you looking well."
Among those he spoke to were Isabelle Thorpe, 23, and Kate Shortman, 22, who are due to compete for Team GB in artistic swimming at the Olympic Games in Paris this northern hemisphere summer.
Both athletes were nominated by Swim England to attend the garden party, and Shortman said the event had left her feeling "very patriotic".
Thorpe told the PA news agency: "He (Charles) did seem like he had a genuine interest and he said he was going to watch our event on TV in Paris, so that would be really cool. Hopefully he does!"
Shortman added: "He was asking us about how long we could hold our breath, which is a very common question."
Speaking to an attendee, the Queen said she was "enjoying the sunshine" and joked with another woman that she had "brought the good weather with you".
Charles and Camilla eventually reunited before spending some time in the Royal Tea Tent, and stopped to chat with more guests before returning inside the palace.
Anne, Edward and Sophie continued to mingle with guests after the pair had left.
The King has been receiving treatment for an undisclosed form of cancer since early February.
On a visit to the University College Hospital Macmillan Cancer Centre last week, Charles spoke of his "shock" at being diagnosed as he met fellow patients.
His appearance at the garden party comes after Prince Harry confirmed he would be unable to see his father during a trip to the United Kingdom due to the King's "full program".
Charles will continue public-facing duties when he meets military staff and their families at a training base for the Army's Royal Engineers on Thursday.
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