The Duke of Edinburgh was already recovering from a hip replacement operation just six weeks before the big day.
He got back in shape for Harry's wedding to Meghan Markle by walking up and down the stairs for days until he was fit enough to attend.
Family biographer Penny Junor described his attendance at the wedding, without even sporting a limp, as "completely heroic".
"Given that he is 96 years old and the operation was just six weeks ago, I think that shows what an extraordinary man he is," she said earlier this week.
"That will have meant a huge amount to Prince Harry. He is enormously fond of his grandfather."
Junor, and the millions of others watching the wedding, would not have known just how heroic the great-grandfather was that afternoon.
His surgery in April followed a series of missed public appearances, including a ceremony at Windsor Castle to formally hand over his role as Colonel of the Grenadier Guards to his son Prince Charles.
He retired from public duties last year – which Buckingham Palace stressed was not health-related – but still accompanies the Queen on some public appearances.
