The queen has become the second longest modern monarch in the world.
He defeated King Bhumibol Adulyadej of Thailand, who reigned for 70 years and 126 days between 1927 and 2016.
Louis XIV of France remains the longest reigning monarch, with a reign of 72 years and 110 days from 1643 to 1715.
The queen, crowned in 1953, became the longest-serving British monarch in September 2015, surpassing her great-grandmother, Queen Victoria.
The 96-year-old celebrated her platinum jubilee earlier this month with four days of parades, street parties and other events held in the UK and the Commonwealth.
He later thanked the nation in a letter, saying it had been “humiliated and deeply touched” and that “this renewed sense of coexistence will be felt for many years to come”.
Mobility problems prevented him from attending many of the events, but he did appear on the balcony of Buckingham Palace at the end of the Jubilee pageant when the holiday weekend was over.
Read more: The Queen’s Journey as a Cultural Icon. The best images of the jubilee across the UK
In his letter, he said: “When it comes to marking 70 years as queen, there is no guide to follow.
“It simply came to our notice then.
“But I was very humbled and moved that so many people took to the streets to celebrate my platinum jubilee.”