She left the car at the top of Castle Hill and was greeted with a rousing rendition of ‘Happy Birthday’ from the crowd.
She then began to greet those who had turned out to see her – some of whom had been camped out for days – accompanied by the Duke of Edinburgh, who worked his way down the crowds on the opposite side of the road.
Among those waiting to see the Queen was Sandra Riley, 57, of Dedworth, who was celebrating her own birthday.
“I came when she was 80 and I gave her some tulips that day,” she said.
“I hope to do the same again today.”
The resident handed her bouquet to the Duke.
The Royal couple made their way down the hill and past the statue of Queen Victoria, the monarch whose record for the longest reign in history was overtaken by the Queen in September.
They then met representatives from the Outdoor Trust, the charity which has created the Queen’s Walkway, a 6.3km (3.9 miles) self-guided walking trail around the town.
Hugo Vickers, a historian and biographer and the trust’s chairman, led the Queen to a panoramic panel displaying the details of the walkway, which she unveiled.
Speaking afterwards Mr Vickers said: “I just think it’s so wonderful to see the Queen on her 90th birthday, looking years younger and so happy, and we are so lucky to have her because we are living in a golden age which she has created.
“I cannot think of another reign in history in which I would have preferred to live.
“It was very kind of her to unveil the walkway today as part of a little ceremonial moment in the middle of a lovely informal walkabout.
“Windsor loves the Queen, the Queen loves Windsor so it all came together in a fantastic way.”
From there, the Royal couple walked up the High Street to the Guildhall, where a party was taking place for others celebrating their 90th birthdays too.
There they met Great British Bake Off winner Nadiya Hussain, who had baked a special birthday cake for the Queen.
She said: “It’s taken five days to make in my kitchen in Milton Keynes.
“It’s an orange drizzle cake filled with vanilla butter cream and orange marmalade, the Queen said it sounded very nice.
“To bake for the Queen was the icing on the cake, excuse the pun.”
Previous Bake Off quarter finalist Martha Collison from Ascot was also at the Guildhall.
On leaving, the Queen and Duke boarded an open-topped car and were driven down Peascod Street, to the delight of the crowds.
Among those waiting to see the Queen were TV stars Sandy and Sandra from Gogglebox, and Joey Essex, of The Only Way is Essex fame, who was sporting a colourful tie given to him by a friend of Princess Diana.
Joining the Queen and Duke on the walkabout was the Mayor of Windsor and Maidenhead, Cllr Eileen Quick, who told the Express the Royal Borough had bought the monarch a special gift.
“We got four dog coats made and embroidered with the Royal Borough crest,” she said.
The Queen was set to light a beacon on the Long Walk yesterday evening.
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