In a message written in a booklet for guests at a Buckingham Palace reception to celebrate organisations helping people with the disease, Charles poignantly reflected on his experience of cancer.
As one of the 1000 or so people in the UK who receive a cancer diagnosis every day, Charles said his journey had given him "an even deeper appreciation of the extraordinary work" of charities.
"Each diagnosis, each new case, will be a daunting and at times frightening experience for those individuals and their loved ones," he said.
"But as one among those statistics myself, I can vouch for the fact that it can also be an experience that brings into sharp focus the very best of humanity."
The 76-year-old monarch, who is still undergoing treatment more than a year on from his diagnosis for an undisclosed cancer, also said his experience had reinforced how "the darkest moments of illness can be illuminated by the greatest compassion".
Charles and his wife, Queen Camilla, invited representatives from cancer charities they support, campaigners and people living with cancer to the royal residence for the reception.
Also present was the ex-wife of Prince Andrew, Sarah, Duchess of York, who has been treated for skin and breast cancer.
"I think the King is a brilliant example of revolutionising patronage," she said.
"He really listens, he wants to fix it, he wants to make a difference."
Though Charles has been increasingly visible in recent months, he was briefly hospitalised in March as a result of "temporary" side effects from a scheduled cancer treatment.
Though no update on the king's health has been provided, his return to public duties has been read that his treatment is heading in the right direction.
"We are so grateful to His Majesty for his continued support, both as our patron and for being so open about his own cancer experience, which has inspired so many others to reach out for support from organisations like Macmillan," said Gemma Peters, chief executive at Macmillan Cancer Support.
Charles' illness came as his daughter-in-law, Kate, the Princess of Wales, was also diagnosed with cancer.
Kate took more than six months off before returning to public duties in late September.
