For her first time at the race course, Meghan presented a trophy alongside her new husband to Frankie Dettori, for his triumph at the St. James's Palace stakes.
Meghan arrived by carriage with Sophie, Countess of Wessex, and Prince Edward, Earl of Wessex.
The Princess Royal and the Duke of York joined the Queen in the lead carriage and were followed by another carrying the Prince of Wales, the Duchess of Cornwall and Princesses Beatrice and Eugenie.
Stella McCartney, who worked with Meghan Markle to create the look for her evening reception, said the dress was the final outing before the former actress began her "very austere" and "very serious" new life. Speaking about their visit, where a group of schoolchildren performed, Andrew Curphey from the Andrew Curphey Theatre Company said: "The Queen said she thoroughly enjoyed it and it was lovely to see the history of the bridge on the videos".
Meghan made another friend in the form of the Queen's bloodstock and racing adviser, John Warren, who has known Harry since he was baby.
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During the process, the hosts went past their own record of 444 runs they put up against Pakistan in an ODI in 2016. Australia skipper Tim Paine opted to bowl first in batting friendly conditions.
Meghan donned a stylish monochrome outfit, designed by Givenchy and completed her outfit with a Philip Treacy hat.
But Meghan opted not to affix her name tag to her dress, instead deciding to carry it around with her handbag.
William and Kate, the Duke and Duchess of Cambridge, did not attend Royal Ascot together until 2016 - five years after they married. The only concession to the long-standing dress code was made in 2017, when women were allowed to wear jumpsuits for the first time.