Queen Elizabeth has overtaken Thailand's King Bhumibol Adulyadej, who reigned for 70 years and 126 days between 1927 and 2016, to set another record.
Louis XIV of France remains the longest-reigning monarch, with a 72-year and 110-day reign from 1643 until 1715.
Crowned in 1953, Queen Elizabeth II became the longest-serving British monarch in September 2015, surpassing her great-great grandmother Queen Victoria. Four days of royal parades, street parties, pageantry and other events were held across the UK and the Commonwealth to mark the Platinum Jubilee milestone.
The Duke and Duchess of Cambridge, the future king and queen, will move to a house on the monarch's Windsor estate this summer. In time, they are expected to move into "the big house" or the Castle, because Charles, the Prince of Wales, does not plan to spend as much time at Windsor Castle when he is king as the Queen does now.