But the Havertown-born entrepreneur is not shy about one thing: her ambitions. After opening seven stores in the Delaware Valley's upscale zip codes, four in the last three years, Mastrilli is planning a "full-on expansion," with a new outlet every three months.
"I don't know if that's fast enough for me," she said in the Germantown Avenue shop, already busy with bargain hunters a day after its grand opening.
How did her Main Line-based company become one of the region's fastest-growing businesses that you've never heard of? One reason may be that while Mastrilli is weak on social networking, she and her family have a strong social conscience. Her sister, Donna - who co-owned the business after their parents opened the original shop in Bryn Mawr - is focusing on a nonprofit offshoot called Greene Street Animal Rescue, a no-kill shelter with a 14-acre farm in Chester County.
But now that she's in charge, Lynne Mastrilli focuses aggressively on growing the for-profit business, hiring two former managers from Restoration Hardware, developing a brand with logos and signs, and eyeing new markets from Cherry Hill to New York to Georgetown.
By the fall, she says, she expects to open the doors on her eighth store.
"If she gets an idea yesterday, there could be a fully operational store in a week," said Donna Mastrilli, 41, still a partner in the business and the lower-key of the siblings.
"I don't have an M.B.A.," said Lynne Mastrilli, a self-described "control freak" who is wearing a flouncy white cotton dress and brown wrap that look as if they came from Greene Street - and scuffed Frye boots, her favorite article of clothing. "It just kind of happens."
That has been true since her parents, avid thrifters and entrepreneurs, opened their first consignment store - then named Renaissance - on the Main Line in 1990. Her father, an architect and building-supply salesman, started it for his wife and youngest daughter, Donna, after he was found to have leukemia.
Lynne Mastrilli, meanwhile, was living in New York, and learning retail on the fly. After passing an Eileen Fisher store, she and her former husband thought they could make their own casually elegant clothes. They bought fabric, designed nine pieces, and he sewed them.
"We got a booth at a trade show and did $100,000," Mastrilli recalled.
Making the clothes was easier than running a fashion business. By 1995 her parents had died, so she closed up shop and came back to help her sister manage the store.
After briefly closing in Bryn Mawr and opening their long-standing shop at Seventh and South Streets, the Mastrillis came back to Bryn Mawr with Lynne helming the city operation and Donna in the suburbs.
By 2008, Mastrilli had "started to get itchy," and in quick order she opened stores in Lambertville, Princeton, West Chester, and Manayunk.
With Donna doing the accounting and bookkeeping and Lynne running the stores, the sisters worked well together, but last year Donna opted out to start the rescue that the animal-loving siblings had talked about for years.
"She never wanted a big company," Lynne Mastrilli says of her sister, who lives on a farm in Phoenixville and tends to her 3-year-old son and a menagerie of farm animals. Lynne Mastrilli lives in a 1960s Spanish-style house in Chestnut Hill - her 10th house in 10 years - with her 11-year-old daughter and six dogs. Houses, not clothes, she says, are her true passion.
After the hectic pace of the last four years, "I was sort of happy to move on," her sister said.
The sisters said they worked well together, mostly because they had separate and distinct roles in the business.
"For anybody to stay in business that long together and, top it off, to be family. . . . It just worked so well for us," Donna Mastrilli says.
Her sister, she says, "just doesn't stop. . . . She's very passionate about the business, and I'm very passionate about the rescue."
They opened the rescue in January with 16 dogs that had been scheduled to be euthanized. To pay for the operation, the sisters plan to sell animal-related products in Greene Street stores and to open another store, Greene Thrift Warehouse in Pottstown, with 15 percent of proceeds benefiting the shelter.
At the Chestnut Hill store, customers eagerly searched through racks of clothes generally priced from $16 to $18, though the huge trough of designer items cost considerably more. Consigners get 40 percent of the proceeds.
With the economy still sluggish, resale is the new black, according to many upscale consignment boutique owners. But Mastrilli's business has always been good. What has changed is the stigma of buying secondhand. "Now it's politically correct," she said.
Nikki Heim, 35, of Mount Airy, is a longtime thrifter and could not believe her good luck when she found the new store.
"I love it, I love it, I love it," she said. "I usually go to Plato's Closet. This is one step up, but the prices are good."
Just as enthusiastic was Cynthia Eckewa 64, of Wynecote, shopping with her daughter.
"I'm going nuts. I'm happy. I'm pumped," she said, her arms full of skirts and tops. "I just happened to be walking by."
Mastrilli was not surprised. She has done this long enough to know she has a winning strategy.
"I could theoretically have a store every 20 miles," she said, an idea that doesn't too far-fetched given her drive. "It's a great business."
Contact Kathy Boccella at 610-313-8123 or firstname.lastname@example.org.