'Les Ameriques' by Kermit Oliver. Each scarf takes almost a year to complete

Kermit Oliver is considered one of the French company's most important creative assets

It's a long way from the Rue du Faubourg Saint-Honoré, just off the Champs Elysée in Paris, to Clifton Street in Waco, Texas, where an elderly postal worker called Kermit Oliver lives with his wife in a modest, prairie-style home. The former is the global headquarters of Hermès, the preposterously expensive French fashion house whose super-chic assistants sell £3,000 handbags, £150 ties, and £100 handkerchiefs to the privileged "one per cent".

The latter, surrounded by potholes and scarred by urban decay, is where Oliver, a 70-year-old African-American, has lived with his family since the early 1980s. After returning from night shifts at the local sorting office, he can usually be found in a 10ft square studio he calls his "monk's quarters".

Kermit Oliver's lifestyle could scarcely be more humble. Yet in the turbo-charged realm of high fashion, where they pronounce Hermès "Err-mezz", members of the cognoscenti consider him one of the £15bn French company's most important creative assets. When he's not sorting mail, the eccentric and somewhat reclusive grandfather toils as a painter. And while he has produced hundreds of paintings and murals over the years, and staged several highly successful exhibitions, Kermit Oliver's real claim to fame involves high-end scarves.

In 1984, a Hermès bigwig chanced upon Oliver's oeuvre while on a driving holiday in the US. He promptly commissioned him to design one of the silk "carré" headscarves which the firm has produced since the 1930s, and which have graced the barnets of everyone from Jackie Onassis and Audrey Hepburn to Lady Gaga and Her Majesty the Queen.

That work, a detailed and extraordinarily beautiful illustration of wild turkeys, mountain lions, longhorn cattle and wild mustangs called "Faune et Flore du Texas" was a huge commercial hit. Oliver was promptly asked to design a second headscarf. Then a third, and a fourth.

Today, Kermit Oliver is the creator of 16 of the most sought-after carrés in the Hermès back-catalogue. Each takes almost a year to complete. Although they retail for roughly £300 – a steep price for what is essentially a square of colourful fabric – his scarves are highly prized by collectors. Second-hand ones fetch £400 on eBay. Vintage examples, unworn and still in the orange cardboard box they were sold in, can go for £1,000.

On paper, that ought to have brought fame and fortune. Hermès carrés are, after all, a commercial goldmine. The firm sells one every 20 seconds, and for decades they have been at the centre of its gilded mythology.

When Jackie Onassis broke her arm in the 1970s, she used a carré to manufacture a sling. When Sharon Stone enjoyed some light bondage in the 1990s movie Basic Instinct, she used one to tie up her partner. But the fripperies of celebrity culture mean little to Kermit Oliver, who has always remained below the radar. Despite his longstanding relationship with Hermès, which has no other US-based scarf designer, he has always eschewed opportunities to raise his profile.

This week, Oliver explained that strange career path to Texas Monthly. Speaking publicly, in his first major interview for seven years, he said that fame simply "doesn't interest me", adding: "Painting is just something I do. I chose not to support my family that way."

Strictly speaking, that's more or less true. A naturally shy man, who has never felt quite at home on the cocktail circuit, Oliver once told a friend: "My needs are very simple. Give me a room with good northern light, my books, my art supplies and a bed, and stick some food under the door, and I'm the happiest man in the world."

Do a little digging, though, and his story starts to look more complex. Oliver's career as an artist spans almost five decades. The son of a cowboy, whose talent was "spotted" at Texas Southern University in the 1960s, he was once feted, in the industry press, as one of the "three most talented" painters working in America.

In the galleries of his native Southwest, his stunningly drafted naturalistic paintings – which often revolve around people, plants and local fauna, arranged in tableaux laden with religious symbolism – have at various points fetched up to $100,000.

Yet two strokes of misfortune have overshadowed his career. The first occurred in the early 1980s, when the economy of Houston, where he had started to build a glowing reputation, cratered during the so-called "oil glut", forcing him to support his family by taking a night job with the postal service.

"He came of age at an unfortunate time," says Alison Greene, a curator at the Museum of Fine Arts Houston, which staged a retrospective of his work in 2005. "The the oil glut in '82-'84 saw the market plummet. That was when he was at his peak." Oliver duly moved to Waco and became a postman.

The second great mishap, which has dominated his existence for the past 14 years, came in 1998 when Oliver's 21-year-old son, Khristian, was among a group of youths caught breaking into an empty house to smoke marijuana. During the ensuing confrontation, a homeowner was shot and killed.

In court, Khristian was found guilty of pulling the trigger. The jury sentenced him to death. After years of legal challenges, which exhausted most of Oliver's life savings, the young man's execution was signed off in 2009 by the Texas Governor, Rick Perry, who at the time was months away from a re-election battle.

Oliver has never quite recovered. "There has been a lot of tragedy in his family, and a turning in and taking care of those closest to him in recent years," is how Alison Greene puts it. Asked about the case by Texas Monthly, he declared: "My son was executed and the Governor was out politicising."

A few years ago, Oliver completed what many critics now call his greatest work: a stunning mural of the risen Christ which adorns the wall of Trinity Episcopal Church in Houston. Laden with symbols of death, redemption and justice, the work uses a likeness of Khristian as the model for Jesus. Proof, perhaps, that behind the effervescent beauty of those Hermès scarves, there is these days an overbearing sadness.