Despite her desire to keep a low profile, Olsen is still very much in the spotlight—even though her role as designer of hers and Mary-Kate's fashion label, The Row, is mainly behind the scenes. Outfitted post-gym in sweatpants, a long-sleeve T-shirt and a The Row cashmere sweater, Olsen definitely looks the part of someone who swapped Hollywood It Girl for New York City fashion designer. She moved to Manhattan at age 18 to escape, studying at NYU and interning for Zac Posen, and it stuck. "I can actually live here," Olsen says. "I don't get followed. In LA paparazzi wait outside my house. I don't leave my house. I can't." Olsen and her sister are no strangers to media attention. From 1987 to 1995 they shared the role of Michelle Tanner on the sitcom Full House. In 1993 they founded Dualstar Entertainment Group—an Olsen twins factory that churns out dolls, video games, perfume, DVDs and a contemporary brand called Elizabeth and James (which debuts a men's line this fall).

With The Row, Olsen is primed to be fashion's next big thing. "It hasn't been easy by any means," she says. "I started with T-shirts. I wasn't sure it was going to become a company. It was more something I was doing for myself creatively." Loads of stars have launched clothing lines—Lindsay Lohan makes leggings, Madonna did H&M. Yet a famous name often isn't enough to keep a label hot, especially during a recession (MT V stars Lauren Conrad and Heidi Montag both debuted fashion lines that quickly vanished). "That's the thing about celebrity brands these days," Olsen says. "People think they can define a brand by just a name, but the product has to speak for itself. It doesn't really matter whose name is on it. That's the truth. It's the work behind it. It makes you step up to the plate."

The Row—named for the bespoke tailors on London's Savile Row—is expensive, sophisticated and sharply cut in luxurious fabrics. Olsen and her team (most of the workforce is under 30) will introduce a men's collection this fall, and she's at work every day from 8 am to 8 pm editing look books, draping garments and running the company. "It's been really interesting to transition into a new business," she says. "I used to be in the entertainment industry and now I feel passionate about the fashion industry. I understand what it takes to make a brand and to have an eye for product that sells. I don't have the time to step away for a month. I hold myself accountable every day." (Would she ever return to performing? "Never say never," she says.)

But even very young moguls need a break, and Montauk is a place where the reclusive Olsen retreats. Weekends are filled with barbecues at friends' houses and dinner at her favorite restaurant, the Beacon in Sag Harbor. "I like to stay in most of the time or go out on a boat," she says. "The water is so beautiful. I like really secluded places. The history and the location of Montauk and the views—it's different. You feel like you're at the end of the world almost. You actually feel like you went away."

This summer she's cohosting a benefit at the Westhampton Beach designer boutique Jimmy's, and a percentage of sales of The Row items will go to the Juvenile Diabetes Relief Fund. "I did it last year," she says. "It's a great cause and I'm so happy they asked me to do it again."

A recent mini-vacation to Jamaica inspired silhouettes for Olsen's next collection. Never without a sketch pad, she drew faces and buildings and lots of scaffolding. "I get really inspired by a person or a personality," she says. "I love drawing, even more when I'm not having to draw a garment." Although Mary-Kate is busy with her acting career, she still helps with The Row. "She couldn't be more supportive," Olsen explains. "From a design perspective she's always bringing concepts to the table."

About every five weeks, Olsen jets to her house in Los Angeles, where she sees her boyfriend, actor Justin Bartha of The Hangover, and checks in at Dualstar. Before she departs she's often especially pressured, supervising fittings in between countless meetings. It's an exhausting routine, and to stay balanced and focused she practices yoga; most recently she attended a retreat to study an Ayurvedic practice called Panchakarma. "It was the first time I had ever done anything like that," she says. "It was really interesting to go by myself and just meditate for a couple of days. I was with two teachers for a week and a half. I needed it." Olsen admits to being a "French fries and hamburger girl," and the retreat taught her how to handle food. "I am literally the worst eater in the world," she says. "Being in the garment district, it's Subway or pizza. I never actually thought about what I eat. [At the retreat] I learned about cleansing and eating right for your body. I came back energized."

Refocused and reenergized, Olsen enters this new phase of her life as calm and collected as one of her yoga poses. She's found a passion in fashion and is eager to see where it takes her. "I wear The Row every day," she says, pulling up the sleeve of her sweater. "I'm able to create clothes that I love and that other people love too. It's been really exciting and rewarding." She takes a deep, confident breath and exhales. "I really just love my job."