According to The Royal Central:
Elizabeth II became Monarch sometime in the early hours of February 6th 1952 when her father, King George VI, died in his sleep. She was in Kenya at the time as she and the Duke of Edinburgh prepared for a royal tour. Prince Philip broke the news of her father’s death to her and she returned to London, as Queen, on February 7th 1952.
The rule that began 69 years ago has gone on to be a record breaker. On September 9th 2015, the Queen became the longest ruling Monarch in British history, overtaking her great, great grandmother, Queen Victoria.
To read the entire article in The Royal Central, please click here.