The 94-year-old monarch was pictured on a 14-year-old Fell Pony called Balmoral Fern over the weekend.
She regularly rides in the grounds of Windsor, which is said to be her favourite royal residence.
The Queen has been isolating there with her husband, the Duke of Edinburgh, 98, and a small number of staff.
The last public picture of the Queen was taken as she was driven away from Buckingham Palace to her Berkshire home on 19 March.
One of her two Dorgis - named Candy and Vulcan - could be seen next to her as they both looked out of the car window.
A Dorgi is a cross between a Corgi and a Dachshund.
The Queen carried out official duties the day before her planned departure, but held her weekly face-to-face audience with Prime Minister Boris Johnson on the phone instead.
The monarch is a passionate horse lover and breeder of thoroughbred racehorses.
Wearing a colourful headscarf and smartly dressed in a tweed jacket, jodhpurs, white gloves and boots, the Queen can be seen in the new photographs taken by the Press Association riding during the weekend's sunny weather.
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