I KNOW WHAT YOU WORE LAST SEASON: October 2010

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It happened by pure accident, claims Alexa Chung of her whirlwind ascendancy to style icon—and now fashion designer. As a TV presenter in her native London, she had to wear a different outfit each episode. "So I acquired a whole collection of clothes I otherwise never would have had access to. It was great!" she says, laughing. With assignments like interviewing Karl Lagerfeld backstage at Chanel, the job also thrust her into rather chic circles. "But when my picture started popping up in magazines, I was like, This is so weird. I just look like everyone else in East London."

Not quite. With her gangly frame and quirky-cool style (a "melting pot" of tomboy, schoolgirl, festival fan, and Betty Draper—thrown together in that effortless way only the Brits seem to truly have a handle on), Alexa has become the Kate Moss of the millennial set. She's a muse to designers and brands round the globe, from Alexander Wang to Mulberry, whose best-selling bag is named in Alexa's honor. When a former flatmate of hers who works at Madewell informed her that her picture was on the label's inspiration boards, one thing led to another, and the model turned TV host turned It girl became a designer. "I'd been offered that sort of thing before," Alexa says, "but it never felt natural. I was quite critical of other 'celebrity' lines, so I didn't want to do it until it was a brand that would totally get my aesthetic." (Luckily for Madewell, Alexa was a fan.)

Needless to say, she didn't take her role lightly. "I'd read this thing about how when [French fashion icon] Lou Doillon did a collaboration with Lee Cooper Jeans, she walked in the first day and gave a denim history lesson, with mood boards and a thousand pairs of vintage trousers. So I was like, I can't let the team down!" says Alexa, who pulled vintage favorites from her closet (i.e., denim hot pants) and took pen to paper to sketch her dream wardrobe: "Things I wanted to exist that didn't," she explains, citing as an example a dress she made for her friend Tennessee Thomas from The Like. The velvet Lolita-style frock with Peter Pan collar "looks like something you wish you'd find in a thrift store but never do," she says. To ensure that the fits were just right (sweaters shrunken, T-shirts oversized), Alexa served as her own fit model. "Nothing felt like work. It was just, 'Let's play dress up!'" she says. Of course, now when she does just that, the inevitable attention paid to whatever she happens to throw on will lead directly back to the source. "I spend my life trying not to get my picture taken, but now I'm going to be like, 'Hey!'" she laughs, pointing at the Alexa Chung for Madewell T-shirt on her back.