And the bride-to-be said she was feeling “wonderful” as she arrived with her mother Doria Ragland at the luxury Cliveden House Hotel after meeting the Queen and Duke of Edinburgh for afternoon tea. The bride and groom-to-be were also present.
Meghan, with an arm briefly around her mother, smiled broadly and looked at ease as they were greeted by the hotel general manager.
The surprise walkabout by the brothers echoed the eve of William’s own wedding in 2011 when the pair greeted royal fans with an impromptu public appearance on The Mall in central London.
Harry shook hands with dozens of well-wishers who shouted “good luck” to the groom.
At one point the prince was handed a small teddy bear wearing a red sweater, which he gave a quick hug before moving on to speak to a young boy sporting a red, white and blue crown-style hat.
When asked how he was feeling, Harry replied: “Great.”
The prince appeared to have a neatly-trimmed beard in preparation for his big day.
Fans, many decked out in the national colours of both Britain and the United States, are camped outside the castle and have been singing loudly in readiness for the festivities.
It was announced earlier that the Prince of Wales is to step in for the father of the bride and walk Meghan down the aisle.
Kensington Palace said that Charles would give his soon-to-be daughter-in-law away when she marries Prince Harry on Saturday in the historic surroundings of St George’s Chapel, Windsor Castle.
The palace said the American former actress had asked her future father-in-law to accompany her as she makes her way to the altar.
In statement, Kensington Palace said: “Ms Meghan Markle has asked His Royal Highness The Prince of Wales to accompany her down the aisle of the Quire of St George’s Chapel on her Wedding Day.
“The Prince of Wales is pleased to be able to welcome Ms Markle to The Royal Family in this way.”
Meghan’s father Thomas Markle was forced to pull out of performing the fatherly duty after undergoing heart surgery in a dramatic upset just two days before the wedding.
Charles previously performed the duty for Alexandra Knatchbull, the daughter of his close friend, Lord Brabourne, in 2016.
A large security operation saw police manning the barricades outside the castle, snipers on the roofs of high street buildings and sniffer dogs carrying out checks before Saturday’s ceremony.