The wind was whipping around a makeshift outdoor SoulCycle studio at Manhattan’s Hudson Yards the other day, but the fashion designer Michelle Smith pulled off her “Legalize Equality” sweatshirt, baring toned limbs. She was hot. For the second time that day, she was front row and center in a spin class taught by her girlfriend, the platinum-haired star instructor Stacey Griffith.

“You are the pebble, you are the water, you are the ripple,” Ms. Griffith said into a headset, as tourists gawked and snapped pictures and Ms. Smith pedaled diligently.

The power couple had more glamorous outings before the pandemic — holding hands leaping into the water on the Côte d’Azur in France last fall, posing for bikini-clad selfies on the beach of Saint Barths in February. But an exercise session in a troubled mall was paradise compared to what Ms. Smith was going through 18 months ago at a corporate office in Midtown.

It was April 2019, and some 20 or so men were bidding for Milly, the contemporary fashion line known for brightly colored, boldly patterned dresses that she had built with Andrew Oshrin, whom she married in 2003 and separated from in 2017.

Carried in Barneys, Saks Fifth Avenue and Bloomingdale’s, Milly had a flagship boutique on Madison Avenue. The brand was a favorite of Jennifer Lopez, Mika Brzezinski and Michelle Obama, who wore a white stretch cotton poplin maxi sundress with a print that conjured the quilts of Gee’s Bend for her official portrait, by the artist Amy Sherald.

But even as the portrait was unveiled in February 2018, showering favor on the designer and the brand, Milly was in trouble.

In its heyday, Milly had generated $50 million in annual wholesale revenue. But costs associated with trying to navigate the changing retail economy had brought it to auction, which concluded with its sale in 2019 to a subsidiary of S. Rothschild & Co., an apparel company, for $5.7 million.

After the last bid, Ms. Smith slipped out of the office, in tears. “I left quietly, not wanting to be noticed. I felt stripped and raw,” she said.

This week, though, she is introducing a new fashion line, named simply Michelle Smith. It diverges from Milly in nearly every way and is a reflection both of the current moment and her own new life.

In the penthouse apartment in Harlem that she shares with her children, ages 13 and 11, and often Ms. Griffith, Ms. Smith described relief from the pressures of the old fashion cycle. “Instead of working from a place of, ‘I need to make a camisole that’s on-trend,’ I am asking myself, ‘How can I express myself most honestly through this fabric,” she said.

Milly was a comer in the contemporary market of the aughts, alongside brands like Alice + Olivia and Marc by Marc Jacobs. It was introduced to a New York defined for women by the ladylike polish of Kate Spade and the lustful adventures of Carrie Bradshaw. The aesthetic of Michelle Smith is that of a more mature New York woman who’s done with norms of office dressing (just let a man criticize her for what she wears to work — not that she’s leaving home to work these days anyway). It is not exactly androgynous, but it is less overtly ladylike. Women won’t wear it to look pretty for others; they’ll wear it to feel comfortable and sexy to themselves.

Bright and flowy dresses have been replaced by comfortable and sexy loungewear: sweaters with extra long sleeves and chill-out slip-on pants, all in cashmere, to be paired and layered with silk camisoles and slip dresses for the dressing-up version of dressing for your couch.

In muted colors (beige, black and a few pops of maroon) the entire new line was hanging on racks in Ms. Smith’s apartment, which doubles as her studio and office. A bolt of black sparkly fabric sat idly in a corner, awaiting a different moment in the culture. “I was excited to use it, then Covid happened and I literally went back to the drawing board,” she said.

Starting a business of luxury casual wear with pieces that cost between $600 to $1,000 during a pandemic marked by a steep economic downslope for the average American isn’t ideal. She is using all her own money to get started, is selling directly to her customers online, and will take pre-orders that will dictate how much she produces.

After decades of the runway-to-department-store churn, Ms. Smith is now interested in conserving resources, both material and psychological. “This is not a time of excess and Michelle’s sensitive to the fact that she is launching a luxury brand when the country is under a lot of strain,” said Stephanie Ruhle, the senior business correspondent for NBC News and the anchor of “MSNBC Live With Stephanie Ruhle,” who has been a friend of Ms. Smith’s and a Milly customer for years.

“People are not going to spend money for the sake of spending money right now. We’ve all trimmed down our lives and so has Michelle. With her, you have a designer that truly lives her brands. Michelle Unzipped” — the Instagram handle adopted by Ms. Smith as she separated herself from Milly — “is the brand I followed much more than a label.”

On that Instagram account, Ms. Smith has chronicled her metamorphosis from creative director of a corporate brand and wife to unbound, freehanded designer and champion of personal freedom, love and L.G.B.T.Q. rights.

Now 47, Ms. Smith first came to New York in 1990 at 18, enrolling at the Fashion Institute of Technology. She’d wanted to be a designer since she was a little girl drawing dresses on the kitchen floor of her family’s middle-class homes in Connecticut, New Jersey and Ohio — wherever she, her siblings and her stay-at-home mother moved for her father’s job as a factory plant manager.

While still a student, Ms. Smith got a retail job at the flagship Hermès boutique on 57th Street which she parlayed into an internship with the brand in Paris. Smitten with the French city and language, she landed another internship at Louis Vuitton and then enrolled at ESMOD, the French fashion school.

The next internship was at the haute couture atelier of Christian Dior on the Avenue Montaigne. Ms. Smith worked on the second floor, illustrating gowns in watercolor: one copy for the client, one for the archives. “It was such a dream,” she said.

Missing the energy of New York, though, she decided to return in 1996.

She got an entry-level job on the design team at Gallery, an outerwear company. “I love coats,” she said. “A coat is the first impression you make.” She was brought in by Mr. Oshrin, an executive on the company’s business side who was impressed by her portfolio.

By 1998, Ms. Smith moved to a design role at Helen Wang, a contemporary brand. “It was a new market sector that I was excited about, with brands like DKNY, Anna Sui and Rebecca Taylor. I wanted to be able to create beautiful fashionable clothing that I could almost afford.”

She carefully tracked the progress of her designs, sold in the department stores that were not yet seriously threatened by e-commerce. “The designs I worked on were doing well and one even got on the cover of a Neiman Marcus catalog,” she said. “It was building my confidence.”

In 2000, Ms. Smith and Mr. Oshrin, who’d begun dating and ideating, started Milly as a wholesale brand. “I handled the design and creative aspects and Andy handled the financial side and production,” she said. The business plan called for Milly to do $1.2 million in wholesale sales in the first year. They hit the target in three months.

“I think Michelle has always done a great job at knowing how to design in a way that is relevant and shifting as things shift in time,” said Tracy Margolies, the chief merchant for Saks Fifth Avenue.

Milly spread across the country, to Neiman Marcus and Fred Segal in Los Angeles. “We were coming out of the minimalist ’90s with the dark Prada and Calvin Klein looks. What I was doing was super-colorful and printed with a little ironic wink to vintage,” Ms. Smith said. “It was totally different from what was going on at the time.”

In 2011, Milly opened its store on Madison Avenue and, a few years later, another in East Hampton. Ms. Smith began to develop close relationships with her customers.

“I would go to the store on Madison Avenue and we would sit in the dressing room and talk about our bodies and our lives and everything women talk about,” said Ms. Brzezinski, who hosts “Morning Joe,” on MSNBC with her husband, Joe Scarborough, whom she married in a dress designed by Ms. Smith. “Michelle can feel your vibe and has an ability to help you translate that into your own personal style that is just so spot on.”

But supplying the contemporary market, which demanded new product every month, could be dizzying. “By the end, I was designing 27 collections a year with over 100 styles per collection,” Ms. Smith said. “It was a crazy carousel and it was going so fast.”

In 2013, the stressed-out designer followed the advice of her friends and started taking SoulCycle classes. She especially enjoyed those of Ms. Griffith, a favorite of Kelly Ripa and the former trainer of Madonna who wrote a book about going from alcohol and drugs to fitness, “Two Turns From Zero.”

“I couldn’t believe Stacey’s energy and personality and the way she lit up the room,” Ms. Smith said. The two became friends outside of class, collaborating in 2015 on a collection of T-shirts with Ms. Griffith’s motivational catchphrases like, “No One Remembers Normal.”

But Milly’s expenses and debt were growing as the brand tried to expand its e-commerce footprint while continuing to meet its department store obligations. Its founders quietly decided to separate while still living and working together, but the situation was untenable. “It just became a dysfunctional environment,” Ms. Smith said, of the company. “I don’t think the right decisions were getting made, because you had one person who said ‘black’ and one person who said ‘white.’”

Mr. Oshrin is currently working as a apparel industry consultant. “It’s a tough time to start any business,” he said, “but Michelle is a talented designer and has tremendous creative instincts.”

Ms. Griffith declined to be interviewed for this article, saying that she wanted the spotlight on her girlfriend.

At the end of 2016, Ms. Smith heard from Meredith Koop, the stylist for Michelle Obama. Ms. Koop had been selecting pieces from Milly for the first lady for years, first buying things off the rack and then working directly with Ms. Smith on pieces like an off-the-shoulder dress Mrs. Obama wore on the cover of Essence in 2016, and a prom dress for Malia Obama. Ms. Smith “is a woman who designs for women,” Ms. Koop said in an interview. “It’s a cliché thing to say, but it’s true in her case.”

The white dress Ms. Koop wanted for the official portrait “was very authentic to what Mrs. Obama would actually wear in her personal life,” the stylist said.

Ms. Smith worked on sketches, adjustments and pulled the dress from her collection to keep it special, but still wasn’t sure it would be selected. “I had made coats for the second inauguration that weren’t chosen, so I didn’t think it was a slam dunk,” she said. Its choosing “was the most exciting moment in my entire design career.” (The dress will be on display in the Smithsonian’s National Portrait Gallery exhibit, “Every Eye Is Upon Me: First Ladies of the United States,” which opens on Nov. 13.)

It is the only piece of her past professional life that Ms. Smith hangs on to, and she finds it irritating when Milly’s new owner seems to claim credit for the dress, as it has on Instagram on occasions like Mrs. Obama’s birthday. “It’s cheesy,” Ms. Smith said. (“We bought all the assets of Milly and that dress is an asset of Milly,” said Mark Friedman, the president and chief executive of S. Rothschild. “I feel bad that she’s irritated, but she shouldn’t be.”)

In August 2018, Ms. Smith was invited to a barbecue in Montauk. Ms. Griffith was there. “We both felt really happy to be in each other’s presence and we started spending more time together,” Ms. Smith said.

When the relationship became serious enough to tell her children, Ms. Smith overheard her son tell a friend, “Wait till you hear this one: My dad has a new girlfriend and so does my mom.”

Last year, the couple made it Instagram-official, posting photos of themselves in embrace at the New York City Ballet. Ms. Smith captioned hers “#lovewins.”

Department stores are falling. Fashion is flailing. Winter is coming. But her wheels are turning, and she finally feels comfortable in her own skin.

“Going through everything I’ve been through, going from a young woman to an adult in my late 40s, I have found my own voice and my confidence to freely express myself in my personal life and my creativity,” she said. “For the first time, everything has aligned and it feels amazing and true.”