The Vogue editor-in-chief opened up about the meeting during an interview with Tina Brown at the Women in the World Summit in New York City on Friday, describing how she, Brown, and Diana met at the Four Seasons just six weeks before Diana's death.
"She looked incredible, first of all. That was the most important thing, and she looked fantastic," Wintour said of Diana, who wore a mint green Chanel suit. "I heard that she spent a lot of time deciding what to wear to that particular lunch."
Brown, meanwhile, was focused on "how terribly lonely" Diana seemed, but also noted "how lovingly she spoke about her boys," Prince William and Prince Harry, who were just 15 and 12 years old, respectively.
Wintour remembered Diana speaking about how uncomfortable the royal family was around the media -- a stark contrast to the Princess of Wales' behavior around them.
"I think that the part of her popularity was that Princess Diana really was so good with the media and was so in tune with them and welcomed them rather than standing back," Wintour said. "In a way, she was certainly the first person in the royal family to understand the benefits of that."
At another point during her interview with Brown, Wintour brought up how Diana's daughter-in-law, Meghan Markle, was also straying from tradition, and "really bringing modernity to the royal family in a way that is inspiring."
"I think the image that I have in my mind that I think so many people all over the world have in their mind of the Duchess of Sussex walking down the aisle by herself," she described. "That, to me, was representative of a modern woman."
"And then looking at her extraordinarily beautiful and proud mother in the pew, to me, that symbolized, 'Goodness, this is going to be a different day for the British royal family,'" Wintour added.
The magazine mogul shared more of her opinions of Markle during a January episode of Vogue's web series, Go Ask Anna, defending her against tabloid criticism.
"I read somewhere that there were members of the royal household that were confused and upset that she woke up so early, at 5:00 a.m.," Wintour said. "She's a normal California girl who gets up early and does yoga and meditates. She also sent a lot of text messages. I mean, what did they expect? That she was going to send messages via pigeon? I think she's amazing."
See more in the video below.