Credit: H&M shirt. Apiece Apart pants. Charvet scarf. Photographed by Carter Smith/Copious Management

Seeing one of the OG ’90s supermodels pop up on the runway these days is the fashion equivalent of the surprise concert cameo, kind of like when Eminem rose out of the floor to take the stage with Drake in Detroit last year. It’s a moment for the young ’uns to see how it’s done.

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“When I come in, there’s a level of excitement because I put in my time,” says Amber Valletta, who walked Dries Van Noten, H&M, Isabel Marant, Michael Kors, and Versace last February at the fall collections. She likens this to when she was first starting out and would see the mood of the room change as soon as Linda, Christy, or Naomi showed up. “It’s like any job when you’ve been there for years. You have seniority, and people respect you differently than when you’re an 18-year-old kid.”

Credit: Stella McCartney jacket and trousers. Janessa Leoné scarf. Dr. Martens boots. Photographed by Carter Smith/Copious Management

Dozens of covers and more than two decades later, Valletta, 43, is as busy as ever. Her latest Big Deal ad campaigns include Loewe, Stella McCartney, Theory, and Anthony Vaccarello, before he decamped to Saint Laurent. She’s also racked up 20-plus acting credits, most recently for ABC’s nighttime soap Blood & Oil. And lately she’s taken up the role of advocate.

Credit: Calvin Klein 205W39NYC shirt and turtleneck. Photographed by Carter Smith/Copious Management

In 2013, she teamed up with Yooxygen to launch Master & Muse, an e-commerce platform featuring eco-friendly designers. (The site is currently in transition.) “I’m trying to educate consumers so they’ll put pressure on brands, and trying to get brands to think about [sustainable fashion] in a cool, sexy way,” she says. Valletta cites Stella McCartney as one designer who’s getting it right with cutting-edge fabrics and shoes with biodegradable soles. “She’s very clever,” says Valletta.

Credit: Baserang overalls. Edun sweater. Hermès scarf. J. Crew belt. Harley-Davidson Footwear boots. Photographed by Carter Smith/Copious Management

Earlier this year Valletta, a Sierra Club ambassador, penned an op-ed opposing the nomination of then–Oklahoma attorney general Scott Pruitt to lead the Environmental Protection Agency. She grew up in Tulsa, and she expressed her fear as a mother—Valletta has a 16-year-old son, Auden, with ex-husband Chip McCaw—over Pruitt’s track record on pollution, among other things. “Speaking up for the truth isn’t always easy,” she says. “It takes courage, but it’s necessary.”

Credit: Miu Miu dress. Janessa Leoné scarf. Levi's belt. Photographed by Carter Smith/Copious Management

Tenacity is a family trait. Valletta’s parents split up when she was 2, and her mom “worked her ass off” to support her and her younger brother, delivering mail and eventually rising through the ranks at the U.S. Postal Service. She too was an activist and helped stop a nuclear power plant from being built near their hometown, getting arrested in the process.

Credit: Bottega Veneta dress. Aigle boots. Photographed by Carter Smith/Copious Management

“She’s one of the strongest and most adventurous women I know,” says Valletta, who adds that her mom made the famous 500-mile Camino de Santiago pilgrimage just six months after surviving breast cancer for the second time. “She freakin’ walked this trail!”

Credit: Dior jacket and pants. Janessa Leoné scarf. Harley-Davidson Footwear boots. Photographed by Carter Smith/Copious Management

Valletta’s own hiking partners include her two dogs, Nila and Gia, who, along with her son and boyfriend, hairstylist Teddy Charles, round out her low-key Los Angeles life. Her idea of the perfect day is simple enough: cooking a great meal, taking a Body by Simone class, and seeing a friend. “That’s big for me,” she says, laughing. “I don’t need a lot to make me happy.”

Fashion editor: Paul Cavaco. Hair: Teddy Charles for The Wall Group. Makeup: Kate Lee for Starworks Artists. Manicure: Morgan McGuire for The Wall Group. Set design: Ward Robinson for Wooden Ladder.

For more stories like this, pick up InStyle's October issue, on newsstands and available for digital download.