She has surpassed her great-great-grandmother Queen Victoria's record on the throne and has reigned for more than 23,226 days, 16 hours and 30 minutes.
The monarch, however, remains modest and matter of fact about the achievement.
Although she thanked the nation for their kind messages as she opened the Borders Railway in Tweedbank on the landmark day, she admitted that the milestone was "not one to which I have ever aspired".
She added: ''Inevitably a long life can pass by many milestones. My own is no exception.
"But I thank you all and the many others at home and overseas for your touching messages of great kindness.''
She is believed to have passed Victoria's record at around 5.30pm but the exact moment she became Queen is hard to calculate as George VI died in his sleep in the early hours of the morning, possibly at around 1am.
At this time of year, the Queen is usually enjoying her much-loved annual break at Balmoral, but she broke from her holiday to carry out duties to mark the historic occasion.
The 89-year-old monarch, with the Duke of Edinburgh at her side, spent the day travelling 40 miles on the railway with Scotland's First Minister Nicola Sturgeon, starting at Edinburgh's Waverley Station, stopping off at Newtongrange in Midlothian before carrying on to the Borders town of Tweedbank.
She delivered her short speech at her last stop, demonstrating her business as usual approach at the end by remarking: "So now to the business in hand. It is my very happy duty to declare the Borders Railway open."
The milestone is tinged with sadness for the Queen as the calculation of the length of her reign is linked to the death of her father George VI, on February 6 1952 when he was just 56.
There was no mention of Victoria in the Queen's address but she was notably wearing her diamond bow brooch, which originally belonged to her ancestor.
She smiled broadly and waved as she was greeted by delighted crowds in Edinburgh where she began her journey on the polished steam locomotive Union of South Africa with Philip, sitting in a private compartment of the ''Pegasus'' Pullman coach at a crisp white linen-covered table decorated with flowers.
In private, the Queen acknowledged the day's significance. Retired train driver Walter Bell, 88, from Edinburgh, who sat with the royal couple for part of the way, revealed: "She said to me we've both got a special day."
Normal service was suspended for a time in the House of Commons as MPs paid tribute, with Prime Minister David Cameron saying: "The Queen is our Queen and we could not be more proud of her.
"She has served this country with an unerring grace, dignity and decency and long may she continue to do so."
Ms Sturgeon told the Queen in a speech: "For those watching from around the world let me say, on their behalf ma'am a simple but heartfelt thank you.''
Afterwards, she described the Queen as being "in tremendous form".