Speaking in the Scottish Borders, the 89-year-old monarch said the title was "not one to which I have ever aspired".
At about 17:30 BST the Queen will have reigned for 23,226 days, 16 hours and approximately 30 minutes - 63 years and seven months.
Dressed in turquoise with her trusty black handbag at her side, the Queen spoke briefly to the gathered crowds on the day she passes the record set by her great-great-grandmother Queen Victoria.
"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 said.
LONG LIVE THE QUEEN