To mark the occasion the BBC opened its morning news broadcast with the national anthem, "God Save the Queen," while the country's foreign secretary Boris Johnson took to Twitter.
"Hip hip hooray for The Queen," he wrote, adding "happy birthday Ma'am!"
This year's celebrations are a notably more low-key affair after a year of festivities to mark her hitting 90.
A thousand beacons were lit around the country on April 21, 2016, and other events last year included street parties and a pageant at her Windsor Castle home.
"I'm still alive," the Queen joked in June during a visit to Northern Ireland, one of the numerous engagements filling the royal calendar.
But the ever-popular monarch has begun handing over duties to other royals, standing down as patron of 25 bodies at the end of last year, including the Wimbledon tennis championships.
While her eldest child Prince Charles is heir to the throne, greater attention has fallen on his two sons William and Harry, who have taken on some of the Queen's duties.
The arrival of Prince William and his wife Kate's two children -- Prince George and Princess Charlotte -- has cemented the country's positive view of the royal family.
Prince Harry is also admired, receiving widespread acclaim this week for speaking out about seeking counselling years after his mother's death when he was a child.
Reforming the royals
"My whole life, whether it be long or short, shall be devoted to your service," she said in a speech to mark her 21st birthday, years before starting her reign.
One of the most testing periods for the monarch came during the 1990s. The Queen dubbed 1992 her "annus horribilis" after the marriages of three of her four children broke down and Windsor Castle was badly damaged in a fire.
Five years later she faced the wrath of the British public when Princess Diana died and the Queen chose to stay at her estate in Scotland rather than return to London.
Buckingham Palace has since been careful to avoid such clashes with the Queen's subjects.
She has remained quiet about political issues such as the divisive Brexit referendum and, in January, Prime Minister Theresa May's controversial decision to invite US President Donald Trump for a state visit.
In modernising her family's image and refusing to become political, the Queen has held on to the nation's affection. While generally in good health, she suffered a cold over Christmas and her recovery was watched anxiously.
Despite being followed closely in her public appearances -- feeding an elephant at a British zoo this month, for example -- she has succeeded in keeping life behind palace doors relatively secret.
While her love of horses and Corgis is well publicised, palace insiders have revealed that the Queen's private pastimes include crossword puzzles and a Dubonnet-and-gin cocktail before lunch.
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