She became queen by a twist of fate and on Sept 9, 2015, Queen Elizabeth II became Britain's longest-serving monarch.
She has reigned longer than her great-great grandmother Queen Victoria, who held the throne for 63 years and 216 days.
Here are 10 things you should know about the Queen:
1. She was born in Mayfair, London on April 21, 1926 as Elizabeth Alexandra Mary Windsor. She was the first child of The Duke and Duchess of York, who later became King George VI and Queen Elizabeth. Called Lilibet by her family, the young princess was not expected to ascend to the throne one day. At the time, she stood third in line of succession after Edward, Prince of Wales (later King Edward VIII) and her father, The Duke of York (later George VI) .
2. Her father took the crown when his childless elder brother abdicated in 1936 - after reigning for just 325 days - to marry American divorcee Wallis Simpson. After her father's death, Elizabeth, then only 25, ascended to the throne on Feb 6, 1952. That made her the 40th monarch in a royal line that traces its origin back to Norman King William the Conqueror, who claimed the throne in 1066 with victory over Anglo-Saxon Harold II at the Battle of Hastings.
3. During her reign, she has witnessed how the world transformed: When she succeeded her father, the world's population was around 2.6 billion people; the Korean War was still under way; the People's Republic of China was only two years old; and Britain was just about to join the United States and the Soviet Union as a nuclear weapons power. Today, the global population has tripled from 1952 to an estimated 7.3 billion; China is an emerging superpower, with the largest economy in the world on purchasing power parity terms; the Soviet Union has long disappeared from the geopolitical landscape; and the United Kingdom has transitioned to what some term as a "middle power" .
4. Since becoming queen, she has seen 12 British prime ministers from Winston Churchill to David Cameron, 12 US presidents from Harry Truman to Barack Obama, and seven popes from Pius XII to Francis.
5. Mr Tony Blair, 62, is the first British prime minister to have been born during the Queen's reign. "The first time she saw (Cameron) he was playing a rabbit in a school production in which her son Prince Edward was taking part," said royal historian Hugo Vickers. "He is the man from whom she now takes formal advice."
6. One of the most difficult times during her reign was the death in 1997 of Prince Charles' first wife, Princess Diana. The Queen was widely criticised by the public for her apparent "business-as-usual" message by taking Princes William and Harry to church at Balmoral, a few hours after Prince Charles had broken the news to them of their mother's death. There were also accusations that the Queen remained in Scotland for too long, and did not return quickly enough to London, which was the focal point for public grief over the Princess' death. Her close friend, Lady Angela Oswald, said the criticisms were unfair. She said the Queen brought her grandsons to church because they needed to find comfort after Princess Diana's death and she remained in Scotland so that she could be with them.
7. The queen's personal fortune, largely inherited from her family, is estimated to be about US$425 million (S$604 million), according to Bloomberg Billionaires Index. That's a mere 3 per cent of the wealth of the richest Briton Gerald Grosvenor, Duke of Westminster, which totals US$14.4 billion. Europe's richest woman Liliane Bettencourt has a US$32 billion fortune.
8. The Queen's love of corgi is well known. Her father introduced the breed to the royal famiy in 1933 when he bought a corgi called Dooki. The dog proved popular with his daughters, and the Queen was given her first corgi, Susan, on her 18th birthday. She has reportedly owned over 30 corgis during her reign - many were direct descendants from Susan. She has also introduced a new breed of dog known as the "dorgi" when one of her corgis mated with a dachshund named Pipkin.
9. Many have described the Queen as serious and "poker face", but those who know her well say she has a sense of humour and a talent for mimicry in private company. Bishop Michael Mann, the monarch's domestic chaplain, once said "the Queen imitating the Concorde landing is one of the funniest things you could see". Mr Ian Paisley, the Northern Irish clergyman and politician, also said that the Queen was a "great mimicker" of him.
10. The Queen married Philip, a distant cousin, in 1947. He renounced his titles as prince of Greece and Denmark and his career in the Royal Navy to be with her. She once described him as "my strength and stay".
SOURCE: AGENCE FRANCE-PRESSE, REUTERS, ASSOCIATED PRESS, TELEGRAPH, THE BRITISH MONARCHY WEBSITE, ABC ONLINE