Mistress of Her New Domain

“Oh my God, that was my first boyfriend’s mom!” said Shoshanna Lonstein, teetering past the tables of Henri Bendel’s tearoom in of-the-moment stiletto heels.

The first thing you notice about the 24-year-old fledgling fashion designer is how stunning she is.

For lunch, she wore dark denim jeans under a lean gray belted coat, and when she reached to hang the coat on the back of her chair, her bare midriff peeked out from under a tight black sweater. She is sexy without hard edges and quite down-to-earth. (She suggested we share a taxi back to the fashion shows at Bryant Park.) Shoshanna, her dress line, embodies the same winning combination of sexy and sweet.

“It’s Hamptons meets California lifestyle,” said Tom Julian, a trend analyst for the New York-based Fallon McElligott ad agency.

Made of pretty fabrics like gingham and Liberty flower prints, her designs use heavy-duty Lycra and corset-like features such as boning to offer support for busty women like Lonstein who would never dream of leaving the house without a bra.

“What I wanted to wear, I made,” she said, nibbling on a cucumber tea sandwich. Lonstein started having many of her clothes custom-made when she was still in junior high at Nightingale, the tony Upper East Side private school, because off-the-rack clothes that fit made her look too mature for her age.

“I knew from myself and my friends that there was this gap in the market, that there were all these people who were in-between sizes,” she said.

If her name rings a bell, it may not be because it graces a fledgling line of clothes, but because her life took a “Lolita”-like turn when, at the tender age of 17, she began dating comedian Jerry Seinfeld, then 37. From the moment they met (he supposedly spotted her in gym class at Central Park), their relationship was first-class tabloid fodder.

Lonstein won’t talk about him at all--even conditioning our interview on the premise that she not be asked about her personal life.

(“Especially with what happened last year,” said Lonstein’s business partner, Felicia Marie Geller, referring to Seinfeld’s nuptials. “It’s just not relevant.”)

“She took . . . her previous relationship and made something of it, which is a good thing,” said Sunset Plaza retailer Tracey Ross, for whom Lonstein worked while a student at UCLA. “But her clothes are adorable in their own right. Every time I have ordered her line, it has sold out,” Ross said.

Lonstein moved home to New York after graduating from college in 1997, and announced to her father, Zach Lonstein, chief executive of Computer Outsourcing Services Inc., that she wanted to start her own lingerie line. Less than thrilled, he suggested she try working in the clothing business first.

She did and produced a sample collection. She and her father struck a deal: He would give her money to launch the business. In return, she would live at home on Fifth Avenue.

She began to design strapless dresses, some with halter tops, that incorporated elements of lingerie, such as boning and corsets. She also came up with the marketing idea, now widely copied, of attaching pouches to her dresses containing a matching pair of thong underwear. The Shoshanna line was launched in November 1998.

Now it is carried in more than 200 specialty and department stores nationwide, including Bloomingdale’s and Fred Segal. Shoshanna grossed $2.5 million last year and has a showroom in Japan.

Lonstein, who writes an advice column for Cosmopolitan, was in the tabloids again recently but not in connection with her famous ex-boyfriend. During a personal appearance in Miami last year, several samples from her line were stolen and later turned up on a woman dining next to Lonstein and Geller at the restaurant Carpaccio. Lonstein demanded the woman return her clothes on the spot and called the police. The woman returned the clothes; Lonstein did not press charges.

Although not amused by the Miami incident, Lonstein said she is flattered when her designs are imitated. The first time she spotted a $58 copy of one of her $130 dresses, she bought it to hang on the wall of her 39th Street office.

“Celebrity got her recognition and visibility, but smart design and networking brought her to the next level,” said analyst Julian.

“I have no idea where I’ll be in six months,” Lonstein said. “Every day I’m excited by what I do, but there are also days when I say, ‘What have I gotten myself into!’ ”

What makes it all worthwhile, she said, is to see someone wearing her clothing. “I just want to attack them and ask, ‘How do you feel in that dress?’ ”