Erin Wasson: The model who’s better dressed off the catwalk than on it

When Erin Wasson arrives in the hotel suite for her photocall, everyone is a little taken aback. By the model's stature, perhaps; by her amber coloured skin, sure; but more because she is wearing a dress. And not just any dress: a floor-sweeping (well, it would be if she weren't wearing platform bovver boots that render her 6ft 4in) oyster silk number, with tiny straps and a cutaway back, exposing yet more of that flawless amber hide.

This is no fashion faux pas; she looks considerably better than anyone I have ever had the pleasure of meeting, but she looks so girly. This is Erin Wasson, the original Model Off Duty, whose idiosyncratic blend of sportswear, grunge and haphazard tailoring started a veritable fashion revolution in recent years. Where are her denim cut-offs, we are all wondering.

"This?" She lifts the hem a little, exposing tanned Twiglet legs. "It's from home, a woman in Texas, but I can't remember her name." Wasson, 27, hails from the Lone Star State, although her rough-dried, dirty-blonde waves and nonchalant leanness speak more of California, which she has since ' made her home. Her every gesture is languid; her progress around the room to greet us all seems effortless and relaxed. She makes a cup of tea, she lights a cigarette, she tells us about her tattoos, many of which crop up in unexpected places – the backs of arms, behind an ear, nestling in her hairline – because Erin Wasson is a consummate professional and a model must appear to be a blank canvas.

At least, most models must. "I've always been a girl with a massive opinion," she tells me. "I want to be in the know. Your typical model maybe shows up and is happy to be manipulated and is happy to ask no questions, and I'm quite the opposite. I'm really particular about wanting to know the essence and spirit of a situation that I'm in."

She has opened shows for Gucci and Balenciaga, been photographed by Mario Testino and Steven Meisel, appeared on the cover of Vogue, Elle, Allure and Numéro – yet the pictures we most enjoy seeing of her remain those of her backstage or walking down the street; in her ripped and faded denim, her leggings, her sloppy jumpers and misshaped grey marl vests. Her defiantly anti-fashion look has become one of the most copied and coveted – not to mention commercialised – aesthetics of modern fashion. She wears what she wants, and now everyone else wants what she wears.

Since Wasson started modelling at the age of 15, when her father sent her picture to a local competition, working standards and roles for models have changed. "The supermodels would show up backstage and be wearing Versace," she explains. "Even on your off days, you looked like you just walked off a photoshoot. Now it's OK to show up in a hoodie and jeans and be comfortable, knowing damn straight you're going to spend your day doing all this over-the-top shit."

Wasson's own look is far from OTT. "I think it's less about being over-analytical with your style or your taste," she says. "You should never think too much about it, you should just let yourself in that moment express what you're feeling. I think that's the most important thing, to find the facet in that moment and express that." Wasson does tend to speak in a rather "Confucius say" sort of way, but she is winsome in her views, articulate, enthusiastic and reasoned. Her practice of Zen and the Art of Under-Dressing is all very appealing, but a strikingly beautiful person in no make-up and rumpled clothes is very different from a mere mortal trying it.

Wasson is in London for the launch of the Maybelline Limited Edition Calendar, a collection of poster-sized shots designed to inspire the beauty industry big-wigs to whom it is sent. It's a sort of industry version of the Pirelli calendar, for which Wasson has also posed and been shot by renowned fashion photographer Patrick Demarchelier. Only 50 of the Maybelline calendars are distributed here, and the imagery is a fluorescent fantasy with Wasson cast as heroine, swinging from gigantic mascara wands or bedecked in orange-and-black checked body paint. The beauty editor of Vogue might receive a copy, say, or the creative director of Elle – all important figures in the exclusive arena of taste-making in an industry reliant on innovation and reinvention.

Wasson, who has been the face of the international make-up brand for seven years, is similar to those figures in her ability to spot rising trends and make them ubiquitous, having started a certain movement within youth culture with her fashion sense and her reclaiming and repackaging of grunge culture. "I'd like to think so," she agrees. "I hope I'm a movement of an open mind. The old-school mentality of approaching fashion in a certain capacity – I just think there's so much more out there. I think the new generation is truly keen on helping each other out and bringing everybody up. We want to build our own mountain and stand at the top and be like, 'Wow, we did this.'"

It's certainly something she has endeavoured, and managed, to do, as muse and friend to New York of-the-moment designer Alexander Wang for several years. His collections of worn denim shorts, slouchy jersey tops and decidedly sexy sportswear encapsulate a modern Zeitgeist – and if they are its wardrobe, then Erin Wasson is its face. "We lived in the same building," she recalls. "It was a serendipitous moment, divine intervention if you want to call it that." I get the impression she wants me to, at least, as she beams at me and rummages in her pouch bag for her cigarettes, never wanting to seem too pleased with herself for having achieved what she has. "Whatever... here I am moving into my apartment in New York and Alex is living on the third floor and we just hit it off and became friends and out of nowhere, he asked if I wanted to style his show."

Wasson styled Wang's spring and autumn 2008 shows, which were inspired by an aesthetic he termed the "M.O.D" – the model off-duty. They were the collections that made him the commercial power he is now, rather than simply one of many youthful breaths of fresh air on the New York fashion scene. Painfully hip, the look was grunge meets Working Girl, a reinvention of late 1980s and early 1990s pieces: shouldered blazers over baggy tanks and crop tops; harem trousers, draped jersey tops and Seattle-style knits. At the tender age of 25, Wang – and his attendant and uniformed hipsters – had not lived through the look the first time around.

When we discuss Wasson's role as muse and stylist, it becomes clear, however, that whatever scene or tastes she and Wang may share, her personal style is something much more organic and less concerted. "For me, it was important to try to fit the outfits to the girls' personalities. We wanted it to translate to the street – these girls are on the runway, but they could walk off and just go about their lives." She references Pixie Geldof, although she struggles with her surname, as an embodiment of the London style she so admires. "I think in America that sort of dishevelled, gritty look seems dirty – like someone who just doesn't give a fuck – and it's not like that. My idea of what is sexy is not the typical idea of American sexy."

It is this more naturally generated sense of style Wasson has made her own. "I think for a while women were dressing for women, and it was a woman's idea of what was sexy, this very done-up look. If you ask men, they're like, 'I love my girl in a T-shirt and no make-up.' I think that's interesting."

Wasson's uniform in pictures – not the ones taken in a studio – is generally jeans, vests and T-shirts, accessorised with skeins of silver chains and dozens of metal bangles like the ones she is wearing today, which make noises like windchimes every time she gesticulates. Which is a lot, because Wasson is a very emphatic person: her idiolect is a strange hybrid of Southern belle politesse and refinement, and downright gung-ho surferisms: "Like, dude!" and "Fuck, man, yeah?" It sounds about right for the sort of girl who has a half-pipe in her back garden, for skateboarding up and down, but who likes wearing floor-length silk evening gowns at 10am on a snowy morning.

However critics term her eclectic chic, they cannot argue with its success. In spring 2009, Wasson launched her own clothing line, Erin Wasson x RVCA (a collaboration with the surfer-lifestyle brand), now in its third season and selling well in boutiques across the world. The year before that, she started her own jewellery company, LowLuv, which specialises in one-off pieces that sell for up to £1,000. She also launched a more affordable costume jewellery range last September. "I feel like everything has been a natural evolution for me," she says simply. "I've always been a creative person but I've just found the right outlets and ways to channel that. So I feel really lucky to be hitting this stride of synchronicity at the moment."

Wasson continually reiterates how she is grateful for the success she has had. "I feel like the luckiest girl in the world. I don't know how I ended up here," she adds. She talks elliptically about her personal "journey" but isn't eager to discuss details: she married an older artist at the age of 17, when she first moved to New York, but the marriage ended in 2003, after which Wasson took five months out, driving a battered truck across America. "I just needed to go where the wind blew me," she told press at the time. Her reticence is perhaps informed by the readiness of critics to pooh-pooh her influence. She is haunted by allegations of hipster solipsism after a comment last year relating to homeless people having "the best style". "The way that they're dressed is so uncontrived," she reasons, "that's the spirit I am drawn to. It's just this crazy concoction that happens naturally and you have to find beauty in that."

This may be a little implausible, but a lack of contrivance is at the heart of Wasson's creative integrity – she calls it "being relatable" – so it is interesting to see how adept she is at manipulating the camera. She knows all of her angles, knows how and when to throw back her head, lift an arm, let a giggle gurgle out, and she knows exactly when to say, "I think we're good. I'm happy with that." With that, she puts down the armchair she is holding above her head and starts asking the room who Cheryl Cole is.

Erin Wasson x RVCA is available at Selfridges; LowLuv is available on shopbop.com