In France, where fashion is considered part of the patrimony and first ladies have been front-row regulars (and supermodels), the government has long treated the industry with kid gloves. Then, in 2017, Brune Poirson arrived.

“In the beginning, everybody thought I was crazy,” said Ms. Poirson, who is officially one of three secretaries of state to the minister for the ecological and inclusive transition and, unofficially, France’s de facto minister for fashion.

Both a champion of the industry and its rare critic, Ms. Poirson, 37, is playing a role in negotiations regarding President Trump’s threats to impose tariffs on handbags and other luxury goods, also known as the “handbag war.” Last year, she also spearheaded wide-ranging legislation that included banning brands from destroying an estimated 630 million euros (or $700 million) of unsold goods annually, a common practice in the industry. Prime Minister Édouard Philippe has said France would be the first to formally adopt this measure.

“When you’re a young woman in government — or in general, in life — and you decide to tackle a topic like fashion, everybody goes after you,” she said in an interview. “It’s almost the end of your reputation. If I were really a politician, I would have taken nuclear energy or something. But I think there is more to do in the field of fashion. I know we need to do something about it.”

It is, after all, France’s second-most profitable sector, worth an estimated €150 billion. (The first is aeronautics.)

And Ms. Poirson isn’t your typical French politician.

“Brune Poirson has a very important mission for the government and for France,” said Pascal Morand, executive president of the Fédération de la Haute Couture et de la Mode, the industry organization for French fashion. “She considers that fashion is essential and should be exemplary, and she brings conviction and determination to promoting the circular economy.”

Seated before a coffee table in a corner of a grand but sparsely furnished ministry office on the boulevard St.-Germain, early in the evening of yet another long day of transport strikes, Ms. Poirson made no secret of her impatience with a sclerotic French system of government and business, or the way things have functioned for years (and even centuries).

“We need to change the way we work,” she said in polished English. “In politics, what’s terrible is that things take time not because it’s difficult, but because people are unwilling to change things for petty reasons. It looks good on TV for a week if you can say you implemented something 10 years after it was planned. It’s completely depressing. So sometimes, if you want to change things, you can’t rely too much on politics.”

Her press officer, seated nearby, squirmed and reached for his smartphone; she burst out laughing. “He’s dying,” she said. “But it’s true.”

She is also nothing if not outspoken. A video shot on the Senate floor in November 2018 captures for posterity her lightning takedown of an older, male colleague. Gérard Longuet, a conservative senator, had addressed her with a patronizing “ma chère amie,” or “dear friend.”

“Call me Madame la Ministre,” she shot back, coolly raising her tone as hoots from other senators echoed in the background.

Ms. Poirson’s clothing style is likewise to the point: She favors minimalism, with the occasional whimsical earring. “I have to be super-simple because that’s who I am,” she said. “No prints, because you get bored and then you want to buy more, and I try to wear exclusively nonsynthetic materials because of all the micro plastics, unless it’s second hand, and clothes that are only made of one fabric.”

In the three years since her appointment, she has championed France’s embrace of a more circular economy, and drafted the zero-waste law that is moving through the legislative system and is expected to be signed by President Emmanuel Macron. In addition to banning incineration of unsold products, the law phases out all use of disposable plastics beginning January 2021; bans microplastics in cosmetics; and makes filters mandatory on industrial washing machines.

Ms. Poirson worked with François-Henri Pinault, chief executive of the luxury group Kering, to shape details of the “Fashion Pact,” an industry initiative that seeks to curtail its environmental impact. The document was signed by 56 companies — though not by LVMH Moët Hennessy Louis Vuitton, the French luxury conglomerate that owns 75 brands — and was presented at the Group of 7 meeting in Biarritz, France, in August.

The pact is not legally binding and was decried by critics as toothless because it lets signatories pick and choose guidelines rather than setting goals, and sidestepped the problem of overconsumption. “Well, it’s the French way of approaching things,” Ms. Poirson said, with the faintest hint of exasperation.

Ms. Poirson has no truck with some luxury brands’ argument that they represent only a tiny fraction of the fashion sector. “That’s like France pointing the finger at China as the bigger polluter,” she said. “I refuse to hear that argument. Everyone should work to maximum capacities to find a solution to climate change. ”She was born in 1982 in Washington, D.C. to French parents — her father worked for the World Bank; her mother restored paintings — and the family returned to France before she could walk. She and her two younger brothers grew up in Apt, in the Vaucluse, part of the then-grittier, pre-Peter Mayle Luberon region in Provence.

She knew from childhood that she wanted to work in the public sphere. But, unlike most French politicians, Ms. Poirson is not an énarque, as graduates of the École Nationale d’Administration, the primary vivarium for French political life, are known.

“I wanted to do the exact opposite of E.N.A. It was a project,” she said. “My roots are really strong. I know exactly where I’m from. It gives you a lot of strength to go anywhere else, anywhere in the world. And I went with the intention of coming back, always.”

That path led to the London School of Economics; Laos — where she spent a year working on education, particularly of young girls, among the Hmong ethnic minority; and, by 2008, New Delhi, where she worked in the public and private sectors, for the Indian cabinet minister Satyan Pitroda and the French company Veolia.

In 2016, she studied political science and sustainability at the John F. Kennedy School of Government at Harvard University, but the combination of the American and French presidential campaigns, Brexit and new motherhood brought her back to France to run for office.

“People often say that when you have a family, you just want to protect them,” she said. “For me, it was the opposite, in a way. I had a daughter, so I had to work twice as hard.”

She ran for local office in the Vaucluse area and used a grass roots campaign to beat the far-right candidate Marion Maréchal Le Pen (niece of Marine Le Pen, leader of the National Front and Mr. Macron’s former opponent). Within 48 hours, Mr. Philippe, the prime minister, invited her to join the government.

Ms. Poirson said her next steps are “not necessarily legislation, but keeping the fight on sustainable fashion.” She said she would like to establish a fund for innovative fashion brands working to change the production system, though has few specifics.

Her position on handbags getting swept up in a possible trade war? “Getting into logistics of commercial sanctions will only create losers,” she said. “Commercial conflicts are just a source of uncertainty and weigh down global growth.”

And she wants to revive sectors like France’s lace industry now hanging by a thread.

“I want to go back to the areas in France with a strong textile history and see how we can actually return to local production,” she said. “I believe in path dependency — when a place is good at one thing, I think you can really rebuild on that.”