Bad weather delayed her arrival at Waverley Station, but the 89-year-old Queen and the Duke of Edinburgh have now set off on the new Borders Railway.
The Queen will have reigned for 63 years and seven months – calculated at 23,226 days, 16 hours and approximately 30 minutes at about 17:30 BST.
David Cameron said the service the Queen had given was “truly humbling”.
Dressed in turquoise with her trusty black handbag at her side, the Queen smiled and waved to those gathered at the station on the day she passes the record set by her great-great-grandmother Queen Victoria
The exact moment the Queen becomes the longest-reigning sovereign is not known because her father, George VI, passed away in the early hours of 6 February 1952.
Business in the Commons was postponed for half an hour so MPs, led by Mr Cameron, can pay tribute to the Queen.
The prime minister tweeted: “Over the last 63 years, Her Majesty has been a rock of stability in a world of constant change.
“It is only right that we should celebrate her extraordinary record, as well as the grace and dignity with which she serves our country.”
Mary Francis, one of her aides in the late 1990s, said the very private monarch could also be “great fun”.
“I’ve seen her on one of the last holidays on the Royal Yacht Britannia – the only time I ever saw the Queen wear trousers.
“She let her hair down. She told stories. She had everybody in fits of laughter so she can be very relaxed – at times.”
Source: 3NEWS.com|Ghana