The monarch spent the day in private at Windsor Castle, in contrast with last year's national festivities.
The Queen, who reached her 65-year Sapphire Jubilee in February, is Britain's longest-reigning monarch.
And famously, on her 21st birthday, the Queen gave a speech from South Africa where she pledged her "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".
- She is the world's longest-reigning living monarch since the death of Thailand's King Bhumibol Adulyadej past year.
On September 9, 2015, she became Britain's longest-reigning monarch, passing her great-great-grandmother Queen Victoria.
The Duke and Duchess of York became King George and Queen Elizabeth (her mother's name as well) after George's brother Edward VIII shirked his responsibilities to pursue a romantic path with American, Wallis Simpson.
Born on April 21, 1926, the Queen usually celebrates her birthday twice: on the day of her actual birthday, and in June during an official military parade known as the Trooping of the Colour.
This afternoon, in honour of her Majesty, the King Troop Royal Horse Artillery fired 41 volleys in Hyde Park.
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Then an hour later the Honourable Artillery Company fired a 62-gun salute at the Tower of London - an extra 21 for the City of London.
After her 90th birthday past year, which saw national festivities held in her honour, the 2017 milestone will seem more subdued.
Her thoroughbred Maths Prize was running at Newbury, and the Queen was seen sharing a joke in the royal box before her horse lined up with six other runners. She is expected to spend the milestone at Windsor Castle.
The Queen also celebrates an "official birthday" which falls on the second Saturday in June.
13 - prime ministers in office since the start of her reign.
Happily for her nation and the world, Queen Elizabeth rose to the occasion magnificently.
Though Queen Elizabeth II's birthday stands a much better chance at seeing sunny weather, the tradition continues.