Roxanne Lowit, an omnipresent fashion photographer whose candid shots of top designers and models frolicking backstage at the world’s fashion shows revealed that the spectacle behind the curtain often rivaled the main event on the catwalk, died on Sept. 13 in Valhalla, N.Y. She was 80.

Her daughter, Vanessa Salle, said her death, in a hospital, was the result of complications of a stroke after a long struggle with Parkinson’s disease.

With her exuberant fashion-week dispatches, party shots and editorial portraits for magazines like Vogue, Allure and GQ, Ms. Lowit captured the biggest names in the industry dropping their guard. Covering the insiders of fashion, she became one herself, along the way gaining access to the stars of art (Andy Warhol, Salvador Dalí), music (Madonna, David Bowie) and film (Robert De Niro, Mickey Rourke).

Seemingly everywhere at once over a four-decade career, she chronicled an evolution in fashion, and fashion photography, from the Studio 54 era of Halston and Yves Saint Laurent in the 1970s through the rise of the so-called supermodels as pop stars in the 1990s to the emergence of designers like Virgil Abloh and Demna Gvasalia in the hip-hop era.

An untrained photographer, Ms. Lowit got her start sneaking past security — posing as a hairdresser, say, while accompanying model friends — to gain access to the behind-the-scenes circus at fashion shows. Making her mark before the age of social media and smartphones, she transcended the paparazzi realm by providing an insider glimpse of A-listers clowning around in evening wear at fashion galas, trying on outfits in hotel suites or hunched in the back seats of limousines.

“It wasn’t easy,” Ms. Lowit said in a 2015 interview with Resource, a photography website. “So I learned to rely on my instincts, dropped names, became friends with the designers and so on.”

Before long, her place among the elites of the rag trade was secure. She was “the invisible of the visual, a witness to the marriage of vanity and fame,” Karl Lagerfeld wrote in a tribute included in “Moments” (1993), the first of Ms. Lowit’s four books. She was, he added, “one who overexposes her subjects while remaining underexposed.”

Even so, with her trademark black pantsuit and her black pixie cut (or, later, her bob), she was highly visible to those in the know.

“I used to see her as this black cat in a corner of the backstage, like staring to everything with bright eyes and jumping on the best scene at the right moment,” the designer Giambattista Valli said in “Roxanne Lowit Magic Moments,” a 2015 documentary.

And she did it in an era when most fashion photographers were men. “Men are pushier than women, especially back when I started in the late ’70s,” Ms. Lowit recalled in the Resource interview. “It was the men with all the bulky camera equipment and safari jackets, pushing and shoving one another to get the best spot, the best picture.”

Instead of force, she leveraged an easy charm and warm personality, the actress Fran Drescher, a close friend and frequent companion at fashion events, said in a phone interview.

“She was so loving, so nurturing,” Ms. Drescher said. “When you connected with her, you knew you were connecting with a real person, not a masked person, an affectation of a person. And that’s unusual in fashion.” Therefore, she added, Ms. Lowit “was trusted by fashion designers, and allowed to go backstage during the big shows. Because prior to Roxanne they never allowed people to see the work in progress, the imperfection.”

As a result, Ms. Lowit’s photographs exposed the humanity and the humor behind the famous faces of fashion, who often seemed frozen in an idealized state in the pages of the fashion glossies. Her backstage photography was “a literal lifting of the curtain,” said Simon Doonan, the former creative director for Barneys New York, who worked on advertising campaigns with Ms. Lowit. Ms. Lowit frequently shot ads, including for Dior, Vivienne Westwood, Acura and Coca-Cola.

She had an uncanny knack for coaxing runway stars to let their hair down, even as it was being done up, Mr. Doonan added: “Backstage at shows, the models would be getting frantically fussed over and primped and tweaked, and she was always able to get them to relax and respond to her with a cheeky insouciance.”

Her most famous photographs have been displayed in museums around the world, including the Museum of Modern Art in New York and the Victoria and Albert Museum in London. One celebrated shot showed Naomi Campbell, Christy Turlington and Linda Evangelista mugging (clothed) in a hotel bathtub. Another captured Ms. Evangelista shadowboxing at a fashion event with Sylvester Stallone. One famous photograph caught Yves Saint Laurent playfully smooching a model of the Empire State Building on a New York sidewalk.

A great photograph, Ms. Lowit told Interview magazine in 2013, is “something that is strong, that has a message perhaps, that’s enlightening, or strange.”

Roxanne Elizabeth Lowit was born on Feb. 2, 1942, in Manhattan, the eldest of five children of Lester and Rebecca (Zuckerman) Lowit, and grew up in the Bronx. Her father worked a variety of jobs, including furrier and taxi driver, and her mother, a pianist who had studied at Juilliard, taught piano.

Her parents split up when Ms. Lowit was in high school, and she moved with her mother to Babylon, N.Y., on Long Island. An arty student who loved to sculpt and paint, she adopted a bohemian look, down to black nail polish, she told Interview.

After graduation, she enrolled at the Fashion Institute of Technology, where she focused on textile design. She spent the early years of her career creating hand-painted silk screens at a textile company, with designers like Anne Klein, Scott Barrie and Clovis Ruffin as clients.

In 1975, Ms. Lowit met her life partner, John Granito, while staying in Mr. Ruffin’s house on Fire Island. (Mr. Granito, a contractor, was building a deck at a nearby house.) In addition to Ms. Salle, he survives her, as do her brothers, Neil and Danny; a half brother, Manny Myerson; and two grandchildren.

Her career took a sharp detour in the late ’70s, when a friend from the Fashion Institute of Technology, the fashion illustrator Antonio Lopez, gave her an inexpensive Kodak Instamatic 110, which she used to take snapshots of her work, as well as the overall scene, at fashion shows.

Her photographs caught the eye of Annie Flanders, the future founder of Details magazine, who was then overseeing fashion coverage at The SoHo News, a downtown newspaper. Ms. Flanders assigned Ms. Lowit to cover Paris Fashion Week.

A photography neophyte, Ms. Lowit learned to load the film for her new Canon A-1 35-millimeter camera on the flight over.

Once there, she wrangled her way backstage at the Yves Saint Laurent show, and she later found herself whisked to the top of the Eiffel Tower with Mr. Saint Laurent and Andy Warhol. Faced with a new career, there was no turning back.

“She thought, ‘This is the life,’” Ms. Salle said in a phone interview. “That’s what made her decide to trade her paintbrushes for a camera.”

Although she revered the work of acclaimed fashion photographers like Irving Penn and Helmut Newton, Ms. Lowit also found inspiration from an unlikely source: Weegee, the New York photographer famous for his shots of crime scenes and the city’s rarely photographed demimonde.

“I’m not into dead bodies,” she said in a video interview for the website Artnet. But, she added, she loved the immediacy and visceral impact of his work with “the people of the night.”

Ms. Lowit inspired others as well, including Warhol.

“Andy once said to me, ‘I learned something from you,’” Ms. Lowit told Interview. “I said, really? What? He reaches into both jacket pockets, pulls out two different cameras and goes: ‘Black and white. Color.’’

Like many photographers of the film era, Ms. Lowit carried both. “He was always observing,” Ms. Lowit added. “Even me, it seems.”