Hundreds of royal fans cheered the monarch as she stepped from her car in front of the Henry VIII gate.
Her arrival was marked by a rendition of Happy Birthday by the Band of the Coldstream Guards.
Born Princess Elizabeth Alexandra Mary of York on April 21 1926 — the year of the General Strike — she was never expected to be Queen.
But she has become the longest reigning and oldest monarch in British history — and now the first to reach 90.
The Queen spent the morning of her birthday in private at her much-loved Berkshire castle — the family home of the nation’s sovereigns for more than 1,000 years.
She went on to mark her special day by carrying out public duties, including opening a walkway created in her honour and carrying out a walkabout through Windsor town.
Hundreds of royal fans, waving flags and clutching flowers, packed the streets to catch a glimpse of the birthday Queen.
She was accompanied by her husband of more than 68 years, the Duke of Edinburgh, who himself turns 95 in June.
Tributes being paid to the monarch illustrate how much technology has changed during her long reign.
Prime Minister David Cameron tweeted: “The whole country will want to wish the Queen a happy birthday today — she has been a rock of strength for our nation.”
Twitter was only a month old a decade ago on the monarch’s 80th, while the first public demonstration of a television came in 1926 — the year she was born.
An official photograph of the Queen surrounded by her five great-grandchildren, including Prince George and Princess Charlotte, and her two youngest grandchildren has been taken to mark the occasion.
Taken by celebrity photographer Annie Leibovitz, it shows the sovereign holding her youngest great-grandchild Charlotte in her arms.
The Queen is also pictured with her corgis and dorgis, and in a third picture with her only daughter, Anne, the Princess Royal.
In the evening, the Queen will light a beacon in Windsor, beginning the first in a chain of more than 1,000 that will spread across the country and the globe.
The Prince of Wales will pay tribute to his mother in a speech, before hosting a lavish private dinner for her family and friends in the castle.
During her 64-year reign, the monarch has been served by 12 prime ministers from Sir Winston Churchill to David Cameron, while Barack Obama, who will drop in for lunch on Friday, is the 12th US president to hold office over the same period.
Over the decades, Britain has undergone major transformations from technological advances like computers and supersonic flight to developments in society and the political landscape.
Her personal life has brought her happiness in her children, grandchildren and great-grandchildren. But she endured an ‘’annus horribilis’’ in 1992, the year Charles separated from wife Diana, the Duke of York split from Sarah, and the Princess Royal divorced Captain Mark Phillips.
She was born on April 21 1926 in a townhouse in London’s Mayfair, the first child of the Duke of York, later King George VI.
Her destiny was changed with the abdication of her uncle, Edward VIII, for the woman he loved — American divorcee Wallis Simpson.
Famously, the Queen dedicated her life to her future role as monarch on her 21st birthday — vowing to serve the Commonwealth by saying: ‘’I declare before you all that my whole life, whether it be long or short, shall be devoted to your service and the service of our great imperial family to which we all belong.’’
A rendition of Happy Birthday erupted from the crowds as the hundreds of well wishers first caught sight of the Queen.
She was dressed in what was described by Buckingham Palace as a “spring grass green” wool and silk weave coat and dress with hat to match, adorned with flowers by her dressmaker Angela Kelly for her birthday appearance on the sunny but chilly day.
Members of the public joined in to sing Happy Birthday as the guards’ band in their red tunics and bearskin hats trumpeted out the tune. At the foot of Castle Hill, the sovereign walked past the imposing statue of her great great grandmother Queen Victoria — the very ancestor she eclipsed as the UK’s longest-reigning monarch last September.
She stopped frequently to collect an abundance of bouquets.
The Queen remarked that it was a “lovely day” as she unveiled a plaque marking The Queen’s Walkway at the foot of Castle Hill during her walkabout.
Hugo Vickers, of The Outdoor Trust, showed the sovereign the plaque which was temporarily standing on the main road through the town centre.
Mr Vickers said: “She was terribly happy with the beautiful weather and the lovely occasion.
“She said it was a lovely day.”
Mr Vickers added that “she loved the panel. Prince Philip said it wouldn’t last long if we left it there.”
The Queen’s Walkway is a 6.3km (4 mile) self-guided trail — mapped out with bronze EIIR crested markers connecting 63 points of significance in the town of Windsor — such as the castle’s Henry VIII Gate and the Guildhall where the Prince of Wales married the Duchess of Cornwall in 2005.
It was designed by The Outdoor Trust to symbolically recognise the moment that Elizabeth II became Britain’s longest reigning monarch on September 9, 2015, after more than 63 years on the throne.
It was a walkabout which also turned into a royal drive-by, when the Queen delighted the crowds by travelling standing up in a review car.
As a finale, she slowly travelled down the street and a side road in the highly-polished burgundy vehicle, smiling and waving to the hundreds of people gathered.
She last used the car for a review of the Royal Regiment of Wales in the Millennium Stadium in Cardiff last year.