Arriving from north Lincolnshire, 73-year-old Karen Maclean said she chatted to King Charles and Princess Kate about her 20-year experience with cancer after the family left their 45-minute service at St Mary Magdalene Church.
Both Charles and Kate have received treatment for the condition over the past year.
Prince William previously said 2024 had been “brutal” and the “hardest” of his life..
Ms Maclean said the trio had a “little talk about cancer” before Kate went in for a hug.
“What a privilege,” she said.
She added that Charles and Kate seemed “very well actually, considering what they’re going through” and said Charles remembered her from a previous encounter.
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“He said to me ‘I can remember you’,” she recalled.
“I’m thinking ‘what?’ Like many years ago?”
Crowds of royalists camped overnight to see the family on Christmas morning.
Retired chef John Loughrey, 69, from Wandsworth, south London, said he travelled by train to King’s Lynn then took a bus, arriving at Sandringham in Norfolk at 7.30pm on Christmas Eve.
“We came here for Catherine,” he said.
“She’s been an inspiration the last year what she’s been through.
“We want to support her and her family.”