After arriving at the Berkshire racecourse in a horse-drawn carriage, the newly-crowned monarch, 74, made his way up to the Royal box with his wife Queen Camilla.
The rarely seen royal was joined by her husband Daniel Chatto, 66, while her brother The Earl of Snowdon rode in the first carriage with the King and Queen consort.
Dutch royals Queen Máxima and King Willem-Alexander are on a three-day state visit to visit Belgium and its royals King Philippe and Queen Mathilde of Belgium.
King Willem-Alexander is expected to make a formal apology on July 1, when the country will mark the 160th anniversary of the end of slavery on what is known as Keti Koti, or Emancipation Day.
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Among those without a definitive burial site include Alfred the Great and the 12-year-old King Edward V. Edward and his brother became known as the Princes in the Tower after they went missing in 1483.
The singer, 65, was one of a just a handful of Australian representatives invited to take part in the Coronation at Westminster Abbey on May 6.