Charles joked that he was "still alive" during a visit with the Queen to east London on Friday, with his cancer treatment set to continue into 2025.
Charles and Camilla attended a reception in the neighbourhood of Walthamstow, and greeted crowds who had gathered outside.
The royal couple attended a celebration of community cohesion at Waltham Forest Town Hall.
Asked "How are you?" by Sikh faith representative Harvinder Rattan, the King smiled as he replied: "I'm still alive."
The King was diagnosed with cancer after receiving treatment for a separate issue.
He had undergone treatment for an enlarged prostate, and while Buckingham Palace has not clarified what form of cancer the monarch has, it is not prostate cancer.
The King underwent treatment throughout 2024 with it set to continue into 2025, according to reports.
The royal couple waved at the crowds outside the town hall and went on a walkabout after meeting representatives from different faith communities in the listed building.
During the visit, Camilla donated 25 toys to Citizens UK, to be passed on to children living in asylum hotels, and a donation to a food bank was left on the King's behalf.
The donation included Waitrose cartons of long-life milk, Christmas puddings, custard and mince pies.
After listening to a performance by a children's choir, A Little Choir of Joy, Charles told them: "I do hope you have a very happy Christmas."
He then joked that the children in the choir, made up of eight to 12 year-olds from schools in the borough, should "make the most of it" for the "poor teachers".
Charles was presented with a Christmas card from the choir, with messages written inside which included "Your majesties, thank you so much for visiting us today" and "hope you enjoyed our singing".
Refreshments for the reception were provided by Haven Cafe, whose owner Usman Khalid is a former asylum seeker who trains refugees to be baristas.
After disorder across some parts of the UK, thousands of people gathered in Walthamstow for a counter-demonstration on August 7, and those planning to protest outside an immigration centre failed to turn up.
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A Palace source told Sky News: "His treatment has been moving in a positive direction and as a managed condition the treatment cycle will continue into next year".
The source added the Royals are optimistic about King Charles’ health, as he continues to undertake royal duties and attend engagements.
Writing at the time of his diagnosis, the Palace said: "During The King's recent hospital procedure for benign prostate enlargement, a separate issue of concern was noted. Subsequent diagnostic tests have identified a form of cancer.
"His Majesty has today commenced a schedule of regular treatments, during which time he has been advised by doctors to postpone public-facing duties.
"Throughout this period, His Majesty will continue to undertake State business and official paperwork as usual."
2024 has undoubtedly been a hard year for the Royals, with both the King and the Princess of Wales being diagnosed with cancer.
Prince William has said that he had "the hardest year" of his life, branding his experience "dreadful".
When asked about his year, William replied: "Honestly, it's been dreadful. It's probably been the hardest year in my life.
"So, trying to get through everything else and keep everything on track has been really difficult.
"But I'm so proud of my wife, I'm proud of my father, for handling the things that they have done.
"But from a personal family point of view, it's been, yeah, it's been brutal."
