In terms of the effect her appearance had on his brand, the designer sums it up as "phenomenal." Speaking to Hello! Fashion, the designer, who has a new collection with British department store Debenhams, said: "The dress she was wearing sold out straight away. We just couldn't make enough, we sold out of everything we had!"
And while Matthew was aware that the style icon might be wearing one of his designs, as he revealed that he had amended it for her ahead of her appearance, he had absolutely no idea that it was going to have such a huge effect.
"It's absolutely incredible," he told us. "It's a huge boost globally, and she just has enormous reach. The recognition of your name goes around the world instantly.
"I don't know anyone else who could do that!"
Speaking about his partnership with the high street brand, which has been in place since 2001, Matthew said: "Debenhams feels like my high street home where we have a formula and it works.
"I love that my brand can reach a bigger audience. I've never been elitist in my approach and in my life, I love a mix of expensive pieces and more affordable items."
So who does he have in mind when he designs? "Although the media tends to think that my customer is blonde, thin and in her 20s, she's actually any age," he said
"My most loyal customer is an 85-year-old woman who lives in the Bahamas and consistently gives us feedback on our collections!" His 68-year-old mom also refuses to wear anything but Matthew Williamson.
The overall vibe of his brand, he told us, is "about celebrating women and optimism and having fun... The clothes are supposed to be about having a great time."
Apart from the Duchess, another famous face who has a huge influence in the fashion world is Kim Kardashian.
But what does Matthew think of her style? "She wears a lot of Balmain – I think Olivier [Rousteing] is a brilliant designer and they've clearly established a great partnership. It's kind of like she's his muse, like Sienna [Miller] was mine."