Charles smiled and waved to passers-by as he was driven the short distance from his Clarence House home in central London to Buckingham Palace on Tuesday afternoon.
The palace has said the King was remaining "wholly positive," and Prime Minister Rishi Sunak earlier said the cancer had been caught early.
Harry, who has fallen out with his father and the rest of the royal family since he stepped down from royal duties almost four years ago, arrived in the United Kingdom to visit his father as messages of support for the monarch poured in from world leaders.
However, a royal source said there were no plans for Harry to see his elder brother, heir-to-the-throne Prince William, during his visit to the UK.
A convoy of cars believed to be carrying Prince Harry arrived at Clarence House in London. (AP PHOTO)
William is expected to step up to fulfil some of the monarch's duties, along with other senior royals while Charles begins a series of out-patient treatments.
"All our thoughts are with him and his family. You know, thankfully, this has been caught early," Sunak told BBC radio, saying he had been "shocked and sad" at the news.
Despite the diagnosis, Charles is planning to continue with much of his private work as monarch including his weekly audience with the prime minister and dealing with state papers.
Sunak said he was in regular contact with the King.
"That will of course continue as normal and we'll crack on with everything," he said.
The cancer was discovered when Charles stayed three nights in hospital last month where he underwent a corrective procedure for a benign enlarged prostate.
Beyond confirming it was not prostate cancer, the palace has not given any further details.
The royal family usually keep medical matters private but the palace said Charles had chosen to go public as he was patron of a number of cancer-related charities.
While the King will receive expert care, his diagnosis will draw attention to the UK's rising cancer waiting times with the state-run National Health Service (NHS) which is widely regarded as being in crisis.
Survival rates for cancer in the UK lag those of other European countries for nine out of 10 of the most common types of the disease, according to an NHS Confederation report published in January.
Harry, who quit royal duties in 2020, jetted back to the UK from California where he lives with his US wife Meghan and two children to see his father after Charles told him and his other immediate family of his diagnosis.
The King's cancer revelation comes as Kate, the Princess of Wales and wife of heir William, recuperates at home after spending two weeks in hospital following planned abdominal surgery for an unspecified but non-cancerous condition.
She is not expected to return to public duties until after Easter and the absence of the senior figures will put pressure on the other working royals to perform extra engagements.
