Charles invited his friend Alan Titchmarsh to “come and see us soon for tea” the day before becoming King.
The prince spent Wednesday working and hosting guests at Dumfries House in Cumnock, the broadcaster and gardener said.
He said that, after dinner at the East Ayrshire home, the heir to the throne and his guests had coffee in a tapestry-lined gallery as they were played “jaunty Scottish airs” on a guitar and penny whistle.
“The prince tapped his foot in time with the music before giving the man on the whistle a bottle of Scotch and the woman on the guitar a bottle of champagne, chatting casually to both,” Mr Titchmarsh wrote in The Sunday Telegraph.
“The recital was short, the applause genuine, and, after making his way around the room, shaking hands with almost everyone there, smiling, thanking them for coming and encouraging them in their endeavours, the prince headed for the exit.
“Because I stood by the door, he shook my hand last of all and said ‘Come and see us soon for tea’. I thanked him and he left the room as Duke of Rothesay, Prince of Wales, Duke of Cornwall and assorted other titles, unaware that he was doing so for the very last time. He was happy; it had been a good day.
“The next day, he woke up early as usual, but by the middle of the afternoon those ancient appellations would be redistributed among more junior members of his family. Prince William became Prince of Wales and Duke of Cornwall. Prince Charles would have but one title: The King.”
King Charles III was formally confirmed as the nation’s new monarch during a meeting of the Accession Council on Saturday.
The Queen’s coffin will begin its journey to its final resting place when it travels from Balmoral to Edinburgh on Sunday morning.
In London, Charles will meet Baroness Scotland, the Commonwealth Secretary-General, at Buckingham Palace, and later host High Commissioners and their spouses, from countries where he is head of state, in the royal residence’s Bow Room.