The King smiled and waved at members of the public as he arrived for the Easter Sunday service at Windsor Castle, in his most significant public appearance since he was diagnosed with cancer.
Charles, 75, joined the Queen and other members of the royal family for the annual Easter Mattins Service at St George’s Chapel on Sunday.
The Princess of Wales and her family are not at the service.
Charles arrived by car at St George’s Chapel alongside the Queen, who was wearing a green coat dress by Anna Valentine and a hat by Philip Treacy, shortly after 10.40am.
A member of the public shouted “Happy Easter”, to which the King gestured with his arm and responded: “And to you.”
The Princess Royal and her husband Vice Admiral Sir Timothy Laurence, the Duke and Duchess of Edinburgh, the Duke of York and Sarah, Duchess of York, all arrived at the chapel a few moments before the King and Queen.
It comes just over a week after Kate released an emotional video message disclosing that she had started a course of preventative chemotherapy.
She, the Prince of Wales and their children Prince George, Princess Charlotte and Prince Louis, who all attended the service last year, are absent.
The family are spending the Easter holidays together as they adjust to Kate’s diagnosis, which was discovered in post-operative tests after major abdominal surgery.
Among those gathered to watch the royal arrivals at Windsor was Anne Daley, from Cardiff, who held up a Welsh flag when the King appeared.
The 65-year-old told the PA news agency: “Did you see the smile (Charles) gave me? He pointed at my flag.”
She added: “He had a lovely smile. He looked well. I think he was happy that we’ve all come.”
The King’s attendance at church will be seen as a move to reassure the public after the shock news about his daughter-in-law.
But the service is a smaller version of the annual gathering, with fewer members of the royal family, as the King has paused public-facing duties while he continues treatment for cancer himself.
He was described by the Palace as being “so proud” of the princess for her courage in speaking out, and is said to be in “the closest contact with his beloved daughter-in-law”.
The King’s own treatment for cancer was announced at the start of February, but he has been carrying out low-key official duties behind palace walls.
Towards the end of February, Charles had an audience with Prime Minister Rishi Sunak and held a Privy Council in his first face-to-face official duties since the diagnosis.
In March, the King held an in-person pre-Budget audience with Chancellor Jeremy Hunt, had a video call with Canadian prime minister Justin Trudeau and had an audience with Baroness Scotland.
Over the course of the month, he welcomed a series of foreign officials to Buckingham Palace, including high commissioners of Jamaica, Tanzania and Singapore.
On Tuesday, as the Palace announced Charles’ attendance at the Easter Sunday service, he was pictured carrying out official duties for the first time since Kate shared her cancer news with the nation, as he met community and faith leaders from across the UK in London.
Ahead of Easter, the King reaffirmed his coronation pledge “not to be served, but to serve” with “my whole heart” in an audio address broadcasted to a congregation at Worcester Cathedral where the Royal Maundy Service was held in his absence on Thursday.