Leaning heavily on a stick, she smiled and waved at the assembled crowd, who sang Happy Birthday, and handed her flowers, cards, chocolates and cuddly toys.
Accompanied by Prince Andrew, she reviewed a march-past by the 1st Battalion, The Royal Regiment of Wales, led by its regimental mascot, Taffy the goat.
Then, with four generations of the Royal Family gathered at Clarence House for a celebratory lunch, the party appeared at the gates with the Queen and her mother at their head.
Prince William, 15, was given pride of place by his great-grandmother when he was at her side to review the King's Troop, Royal Horse Artillery.
And 12-year-old Prince Harry was also present, with his father, the Prince of Wales.
The Queen and Princess Margaret arrived separately just before 1pm, preceded by Viscount Linley together with his wife, Serena.
Also present were the Princess Royal, her husband Captain Tim Laurence and her children Peter and Zara Phillips, with Lady Sarah and Daniel Chatto.