King, 75, left the Easter Matins service at St George’s Chapel in Windsor alongside the Queen before engaging with the crowd outside.
Despite medical advice to limit contact during his treatment, he greeted and spoke with people who had gathered to support him.
A Palace source confirmed that this public engagement was a positive development in his treatment and hinted at potential future public duties.
The King's response to treatment has been encouraging, leading to adjustments in his engagements.
The royal family, including Princess Anne, the Duke and Duchess of York, and the Duke and Duchess of Edinburgh, were present at the service.
The King's presence at the Easter Sunday service followed a pre-recorded audio message at the Royal Maundy Thursday service.
The King and Queen are expected to go on their annual Easter break following the festivities.
Despite their cancer treatments, the royal couple has been committed to their state duties to reassure the nation.
Their public interactions are limited due to medical advice.
The Archbishop of Canterbury praised their courage and selflessness in sharing their diagnoses during his Easter sermon.
The King's brief walkabout after the service was his first public appearance post-diagnosis, where he interacted with well-wishers who shared their support and encouragement.
The royal family then returned to Windsor Castle after the service, signaling a cautious but hopeful outlook for future engagements.