The catwalk season has begun, and there is a new It girl in town. She is Parisian, with dark hair that tumbles in that photogenically messy way that only French women ever master. She wears a lot of black (jeans, ankle boots) and not much makeup – just a perfect red lip – and great perfume. Her look is timeless, understated and perennially chic. Think tailoring, iconic accessories and beautiful silk shirts.

But Marie Blanchet is a whole new breed of fashion leader, because her look is not about new clothes. As the CEO for William Vintage, the London-based vintage label that dressed Adwoa Aboah for the GQ awards last week and sourced the 1960s Courreges trapeze coat that Meghan, the Duchess of Sussex, wore for her New York baby shower, Blanchet stands for a coming age of fashion in which the kudos of the perfectly sourced secondhand piece outshines the appeal of new clothes.

Blanchet’s masterplan is to make William Vintage, which was founded a decade ago by the fashion connoisseur and dealer William Banks-Blaney, the first market-leading fashion brand that doesn’t make any new clothes.

From left: Norman Norell 1965 sleeveless evening dress, Azzaro floral skirt and matching bustier top and Jacques Heim 1955 haute couture beaded dress. Composite: William Vintage at Matchesfashion.com

“Our ambition is to make a look for the contemporary moment, but using vintage clothes,” she says. In an industry struggling to reconcile the environmental impact of clothes production with a fashion news cycle that relies on new collections to drive desire, Blanchet sees an opportunity to create a fashion brand out of pre-loved clothes. The sustainable credentials of vintage fashion are already beginning to make it an alpha choice. Amal Clooney chose a caped ivory silk gown from Jean-Louis Scherrer’s 2007 collection for a black-tie event at Buckingham Palace earlier this year.

“Even five years ago, vintage was a niche thing” says Blanchet. “The interest in sustainability has been tremendous because women are more conscious of the impact of what they buy. The importance of caring for our planet has democratised secondhand fashion.” Vintage enthusiasts are pushing back against disposable culture. “We elongate the life of clothes, by giving them a second life,” says Blanchet. And a well-chosen, decades-old secondhand dress may be a better investment, that will last longer in your wardrobe, than an off-the-rack high-street dress on sale today. “In the past, clothes were made and crafted to last, not to wear for only one season,” she says.

Crucially, vintage no longer means retro. If the notion of a vintage red-carpet dress conjures up visions of dusty corsages and twee satin sashes, take a look at the snake-hipped, emerald-sequinned one-shoulder sheath Aboah wore last week. Only the nerdiest of fashion-watchers would have spotted that the dress was 15 years old, from Tom Ford’s final collection for Gucci. The “ultra-feminine, but modern and chic” silhouette gives the dress the contemporary aura Blanchet looks for. “I would never choose a piece that looked old, or dated. Believe me, I am as put off as anyone by the idea of looking retro.”

Blanchet fell for vintage as a teenager in Paris. “When I was 15, an age when you are trying to find yourself, [watching] the John Cassavetes film Opening Night and discovering the style of Mick and Bianca Jagger – those were huge inspirations to me.” The flea markets of Paris remain her favourite shopping haunts. “French women have a notion of heritage because they learn to dress from their mothers, so they are not put off if a piece doesn’t look new. They have a taste for a piece that has a story, that has lived a life already – “un vécu”, as we say in French.”

Christian Dior sequin feather sleeveless dress, Gianni Versace Couture ruched slit-front gown and Thierry Mugler 1980 deep V-neck cocktail dress. Composite: William Vintage at Matchesfashion.com

Three years at Vestiaire Collective, the leading online resale site for designer fashion, honed Blanchet’s style instincts. Her mantra for vintage shopping is to look for clothes that look contemporary, and align with your personality. She mixes vintage with modern pieces, preferring to buy footwear new – designer heels for evening, Converse hi-tops for day – and scours vintage stores for white cotton blouses and shirts, never buying them new. “One of my biggest obsessions is my collection of Edwardian white cotton blouses and 1900s men’s shirts,” she says. Sizing, she admits, can make vintage shopping hard work. “A French size 42 from the 1970s is the same as a French size 38 now,” she explains.

William Vintage operates at the high-end apex, with an edit currently on Matchesfashion.com that includes an Yves Saint Laurent 1968 Safari suit for £25,000. The partnership with the online retailer, which sees treasures such as a 1993 Gianni Versace flame-red bustier dress (£1,650) and a Thierry Mugler velvet and lamé mini-dress (£1,575) curated under the “brand” of William Vintage, is part of a strategy to position it as a fashion leader, alongside top catwalk designers. But sustainability in such a glamorous light may sprinkle stardust on the more affordable ranks of secondhand boutiques. The Oxfam-led Secondhand September, which hopes to inspire shoppers to buy only secondhand for this month, is intended to mimic the success of Dry January in changing the behaviour of consumers who wish to wean themselves from a dependency on fast fashion.

For Blanchet, the most important part of the appeal of vintage fashion is not how ethical it is, but just how fabulous it is. “Of course, we are sustainable,” she shrugs. “By definition we are sustainable. It’s not even something we have to defend.”