Upstarts! That was this reporter’s initial reaction on hearing that Mashburn, an arriviste from the hinterlands (Atlanta), had the effrontery to open a combined men’s and women’s clothing and accessories store on a busy corner of Madison Avenue just two blocks north of Ralph Lauren’s Rhinelander Mansion flagship and directly across 74th Street from a mega-size Apple store.

Location, location! But what does this newcomer offer? What’s behind the plate glass wraparound windows that practically reach the sidewalk, tempting passers-by to step inside and check out the goods?

The offerings are low-key, unpretentious; they don’t scream excess. They’re excellent examples of updated classics. The men’s Ivy sport jackets and the women’s shirtdresses are arguably among the biggest hits, especially at a time when “there’s plenty of evidence that people are looking for stability in their lives and wardrobes,” said G. Bruce Boyer, a men’s fashion writer and journalist who is a Mashburn customer. “Think of the words most used in fashion now — vintage, heritage, curated, sustainable — and updated classic becomes the perfect choice for today.”

Before Madison Avenue, Mashburn had already opened stores in Atlanta, Dallas, Houston, Los Angeles (currently men’s wear only) and Washington, D.C., each with “a proven track record,” said Chip Ervin, the owner of Grady Ervin & Co., a shop in Charleston, S.C., which stocks the brand’s men’s wear. “The logical extension was New York.”

And New York, said Sid Mashburn, who owns the business with his wife, Ann, “puts you on a different level. I dreamed of New York my entire life.”

The stock-in-trade, which the two design as well as gather from other sources, is rife with carefully thought-out, quietly luxurious details (luminescent Trocas shell buttons, beautifully scaled pockets, flat-felled seams, pick-stitched corners), along with made-to-measure and tailoring services. It’s all done in top-notch fabrics and put together with a measure of sass (as in the Fair Isle-esque wool socks, $35 to $45, and the faux fur bucket bags, $150).

The men’s section has a clubby atmosphere, with a zebra-hide rug and a billiard table. The wares run the gamut: natural-shoulder, close-to-the-body men’s suits, sport jackets (the biggest sellers), shoes, leather goods, watches — one of which, endearingly enough, is the Timex Easy Reader ($55), with a metal band that Mr. Mashburn suggests replacing, when needed, with a nylon NATO strap ($15) — the kind that was issued by the British Ministry of Defense in 1973.

The men’s suits and jackets “are really closer to what Paul Stuart used to do than to Brooks Brothers,” Mr. Boyer said, noting “the soft construction in the shoulder and chest but with some subtle waist shaping, more sophisticated than the traditional Ivy style of tailoring.”

The Virgil sack suit jacket — it got its name from Sidney Poitier’s role as the homicide detective Virgil Tibbs in the 1967 film “In the Heat of the Night” — is elegant but relaxed ($1,350 to $1,950). The unlined, unconstructed Butcher jacket comes in a range of fabrics, including an easy-going, cardigan-like wool flannel ($995).

Ms. Mashburn’s section of the shop, with its gilt-edged glass jewelry case and toile-covered wall, runs its own gamut of goods: outerwear, dresses, separates, PJs, gloves lined with rabbit fur, mink headbands and faux fur phone cases. One standout item is a kimono-sleeve wool tartan coat ($795), which Louise Paul, an accessories designer, called “versatile and funky” when she stopped by.

For this reporter, the most “signature” of the women’s items are the shirtwaist dresses in cottons and silks ($350 to $700). And as if channeling the late-midcentury era that such items evoke, Ms. Mashburn wears her hair in a French twist — in evidence when she and-or Mr. Mashburn are in town to commandeer the shop every few weeks.

White cotton and silk shirts ($195 to $425) go well with a slim black knit pull-on skirt ($250) or a very 1950s Black Watch tartan skirt in bias-cut stretch wool ($395) and the block-heel buckle shoes that are Ann Mashburn mainstays. Available in varying heel heights ($295 to $350), they recall not so much Pilgrim footwear as the Roger Vivier pumps Catherine Deneuve wore in “Belle du Jour.” (The pumps are perfect with the sheer nylons that are back in style or with socks of any length.)

The Mashburns, both of whom are 62, bring style pedigrees to their business. Growing up outside Jackson, Miss., Mr. Mashburn watched “a lot of TV,” he said. “Get Smart,” “I Spy,” “The Man from U.N.C.L.E.” Inspired by “the ’60-style lean tailoring,” he had his trousers tapered (by his mother) when he was a teen. Arriving in New York in 1984, he worked at Frank Stella, British Khaki and Robert Leighton before becoming J. Crew’s first men’s designer — and that, despite zero formal training.

What’s more, that was back in J. Crew’s heyday. In 1991, he joined Ralph Lauren’s Polo design team. Somewhere along the way, he began wearing colorful African neck beads, which he continues doing to this day. They’re sold at the shop ($20 and $25 per strand).

Ms. Mashburn, with an upbringing in Midwestern states, recalled being inspired by iconic pieces of clothing seen in movies: Ms. Deneuve’s trench coat in “The Umbrellas of Cherbourg,” Jean Seberg’s striped Breton T-shirt in “Breathless.” (Hmm … all these ’60s film references.) She, too, moved to New York in 1984, landing a job at Vogue as assistant to the fashion editor Polly Mellen.

“My side of the brand is informed by that,” she said. “It was like going to Harvard for fashion — and life, to be honest. The older women I met taught me about confidence and style and dressing for practicality without sacrificing a sense of imagination.” Next came an editing job at Glamour, followed by styling work at J. Crew. The two met, independent of work, in 1985.

In 2007, by then married, they relocated to Atlanta. “We heard it was a great city, a complex city, with a fantastic food and music scene,” Mr. Mashburn said. There, he quickly found a master tailor, Quang Dau, and opened a men’s wear shop, taking what he described as “an open-kitchen approach,” with the cutting and tailoring done in full view on the store floor.

In 2010, Ms. Mashburn opened her own shop. In 2012, she moved to a larger space that was connected with her husband’s. In addition to the styles they design, they stock carefully chosen pieces from, for instance, Ulla Johnson and Aspesi, for women’s wear; Filson, for its tote bags; Vintner’s Daughter, for cosmetics; and Craighill, for objects for the home.

And while they were at it, the Mashburns raised five daughters, all of whom have contributed to the family business. Elizabeth Mashburn Suarez, the eldest, 33, “helped define the look-feel-voice of the brand from the outset, and we cooperate on all things creative,” Ms. Mashburn said. “Working with my daughters is definitely less complicated than working with my husband!”

The Madison Avenue shop is welcoming and relaxed. A phonograph (yes, a phonograph) plays ’60s and ’70s rock and soul — Fleetwood Mac’s “Rumours,” Al Green’s “Greatest Hits,” Lynyrd Skynyrd’s “Skynyrd’s Innyrds.”

So you wonder: Is this store in some kind of post-midcentury time warp? What is its appeal to customers whose likeliest ZIP codes are adjacent to Central Park, just footsteps away.

“At first I was underwhelmed,” Ms. Paul said. “The overall impression I had was utilitarian. But I got curious, tried on a few things and realized they make their understated statement using high-end fabrics, with subtle details and sometimes daring colors and prints. They have to be sought out. Do busy New Yorkers give them a chance? My guess is yes.” The goods are, she said, solid investment pieces.

“Sometimes boutiques can seem intimidating,” said Laura Cantral, an environmental protection and conservation consultant who is a customer at the Atlanta store. But the Mashburns, she said, “know how to strike the right balance, giving you space and assisting you with what you need. Warmth is their special sauce.”

Contributing to the hospitality are the shop’s staffers — cheerfully unobtrusive aunt-and-uncle and cool niece-and-nephew types who happen to be styling whizzes. “The way we hire is the way it was done in ‘Ocean’s 11,’” Mr. Mashburn said, referring to that film’s recruits for a Las Vegas casino heist. What’s needed, he said, “are the Alan Arkin and the Bernie Mac and the Matt Damon. We like the mix.”

Tony Brand, the men’s department manager, has a past not in crime, but at Prada, Tom Ford, Ralph Lauren and Anderson & Sheppard, the bespoke tailors on Savile Row.

What does Mashburn’s future hold? A Nashville store is in the works for 2024, and under discussion, Mr. Mashburn said, are more openings — in Charlotte, N.C., and Chicago — not to mention a possible outpost in Greenwich Village. On a small scale, the plan seems to be to make the Mashburn brand of classic goods and personal service available countrywide — even in downtown New York.