The King made his first major public appearance since his cancer diagnosis was first announced in February.
Charles, 75, was joined by Queen Camilla as they attended the Easter Sunday service at St George's Chapel at Windsor Castle.
The King has reduced his public-facing duties while undergoing treatment with a Buckingham Palace source saying that he has "responded to treatment very encouragingly over past weeks".
Sunday also featured other significant moments. Firstly, the service was a smaller version of the annual gathering, with fewer members of the royal family in attendance.
The Prince and Princess of Wales and their children Prince George, Princess Charlotte and Prince Louis, who all attended last year, all missed the church service on Sunday.
The event came just over a week after Kate, 42, released an emotional video message revealing that she has started a course of preventative chemotherapy.
The Wales family are spending the Easter holidays together as they adjust to the Princess' cancer diagnosis, which was discovered in post-operative tests after major abdominal surgery.
Other notable absences were Mike and Zara Tindall, with sportswoman Zara taking part in an equestrian event at Thoresby Hall in Nottinghamshire.
The King was joined by his siblings and their respective spouses, the Princess Royal and Vice Admiral Sir Tim Laurence, the Duke and Duchess of Edinburgh, and in another significant move, Sarah, Duchess of York joined her ex-husband, the Duke of York, at the service for the first time since their divorce in 1996.
Sarah and Andrew have remained on amicable terms since their initial separation in 1992 and still live together at Royal Lodge in Windsor.
The Duchess, who was herself diagnosed with skin cancer this year after facing breast cancer last summer, last attended the Easter Sunday service in 1991 alongside her friend and sister-in-law, Diana, Princess of Wales.
It comes after Sarah joined the royals on the walk to church on Christmas Day last year for the first time in over three decades.