The King met the England manager Gareth Southgate as he and Queen Consort had their first official outing since the release of Prince Harry's memoir.
The King met the Three Lions boss at Norbrook Youth Club in Manchester to hear the life stories of young people supported by his charity, the Prince's Trust.
The youth club was where Manchester United star and home-town hero Marcus Rashford played pool with his friends when not training with his club.
The 25-year-old England international was praised by the Queen Consort, who unveiled a donation of books from the striker.
Camilla described the player's gesture as "wonderful", while his international manager Southgate described the forward, who has won praise for his campaigning on youth issues, as "impressive".
The meeting was part of a whistlestop tour of Greater Manchester by the King and Queen Consort, their first public outing together since the release of Prince Harry's memoir Spare ten days ago.
Earlier in the day, the pair waved to gathered crowds outside Bolton Town Hall on Friday morning before the King visited the world's biggest Corn Flakes factory in Manchester.
Charles joked he was a year late to visit the factory, which celebrated its centenary in 2022.
The Manchester site is the biggest cereal factory in Europe, producing breakfast favourites including Corn Flakes, Frosties, Rice Krispies, Crunchy Nut and Coco Pops.
The King unveiled a plaque marking his visit, celebrating 100 years of "bringing breakfast to Britain".
He sparked laughter when he said: "Sorry I'm a year late."
During his visit to the plant, which has just under 400 workers, the King watched cooking demonstrations and saw the various cereals on offer.
He heard about efforts to reduce salt and sugar content and asked about the popularity of Coco Pops as he stood in front of a strawberry and white chocolate version of the cereal.
At one point as he chatted to workers, he said he thinks molasses is "very special".
The King wore a white coat for a walk around the factory floor, where he stopped and chatted to staff.
Kellogg's has been a holder of the Royal Warrant since the reign of King George VI.
Cereal from the business was historically delivered to Buckingham Palace in a small van called Genevieve.
The King also met a group of interns and engineering apprentices during his visit, which lasted approximately an hour.
After the King left Chris Silcock, Kellogg's UK and Ireland managing director, said it was "fantastic" that he had visited.
"It doesn't get any better than that for us," he said.
Mr Silcock said he found the King "so warm and engaging", adding that he was delighted to see the King's "focus on people", meeting apprentices and workers.
Earlier on Friday, the King visited GCHQ's North West hub.