Charles hosted a reception in central London to highlight the work of nurses and midwives over the decades as part of the NHS 75 celebrations.
The King took the time to meet many of the guests and the crowd around Charles was sometimes five deep as he made his way around a ballroom.
He appeared to enjoy himself as he moved around the room making jokes and thanking nurses for their work.
Charles was given a birthday card on behalf of a six-year-old girl with whom he shares a birthday.
Sandra Stephens, 36, handed the King the letter on behalf of her daughter Isabella.
She said: “It is my daughter’s birthday today as well so she wrote him a letter and I offered him the letter.
“It said, ‘Hello, King. Do you know who I am? My name is Isabella and it is my birthday as well so happy birthday, too.’
“He looked at the letter briefly and I asked, ‘Could you accept this letter?’ And he said, ‘Of course’. He was just so pleasant.”
Charles also stopped to practice his Swahili with a Kenyan nurse.
Bernice Boore, 47, who also spoke to the King at a reception last month, said: “It was really interesting to see the King remembered me from the conversation we had last time.
“We talked about Kenya and I asked him if he enjoyed (his recent trip there).
“Then we started talking about Swahili words. I think he knew his Swahili, to be fair. That was really exciting.”
The King greeted the Health Secretary at the start of the event. After speaking to Charles, Ms Atkins said: “What an amazing end to this part of the day, to be here in Buckingham Palace with His Majesty the King, celebrating these incredible people who work in our NHS.
“I think it is so wonderful for His Majesty to show his appreciation, and therefore our appreciation, for all that they do to look after us.”
Asked if the King congratulated her on her new role, Ms Atkins said: “He was as welcoming and kind and friendly as one would hope to imagine.”
The King was also treated to a surprise performance of Happy Birthday by the NHS choir, with nearly the whole ballroom joining in, before he left the event.