The Duchess slipped in and out the underground car park of the Dorchester Hotel inside a blacked-out people carrier, in the hope her presence would not be noticed.
The Mayfair hotel has been the focus of a worldwide boycott backed by the likes of Stephen Fry, Anna Wintour and Sir Richard Branson. Kim Kardashian canceled bridal shower at the venue as well.
Celebrities have voiced their opposition to harsh new Islamic laws in Brunei that have increased the punishment for homosexuality to death by stoning.
The civil marriage ceremony the Duchess to have attended was that of her first cousin Adam Middleton to the interior designer Rebecca Poynton.
Speaking to the Daily Mail's Sebastian Shakespeare, who broke the news of the Duchess secret visit, an insider said: "The wedding was a big boost for the hotel... It shows that the royals will not let the Brunei business keep them away."
Throughout April and early May, at least 20 events were canceled at the historic hotel, with stars including Jay Leno and Ellen Degeneres taking part in protests outside the pink stucco building.