Patrick Demarchelier, one of the photographers whose work helped define fashion and celebrity in the late 20th and early 21st centuries, died on March 31. He was 78.

His death was announced on his Instagram page.

The personal portraitist of Diana, Princess of Wales, and the first non-Brit to become an official royal photographer, Mr. Demarchelier was most famous for his work with Vogue and Harper’s Bazaar, and he was the subject of a major bidding war between the glossies. Indeed, he became so synonymous with Vogue that his name made a cameo in “The Devil Wears Prada.” “Get me Patrick” was a much-quoted line from the film.

“Patrick takes simple photographs perfectly, which is of course immensely difficult,” wrote Anna Wintour, the editor of Vogue, in a 2015 essay for a Christie’s auction of his work. “He makes attractive women look beautiful and beautiful women seem real.”

An ability to combine both ease and elegance set his work apart. His photographs of Diana often had an unstudied aspect that crystallized the princess’s informal personality, such as a snap of her taken in 1990 sitting on the floor in a strapless white gown and diamond tiara, hugging her knees. A photograph of Madonna for the cover of Vogue in 1989 captured her in a white bathing suit laughing and looking over her shoulder in a pool, as though she had just popped up from a swim.

“I like to do the pictures before people get too self-conscious,” he told the actor Keira Knightley in Interview magazine. “I like to be spontaneous and get a shot before the subject thinks too much about it.”

Born in 1943, Patrick Demarchelier grew up in Le Havre, France. With no formal training in photography, he started taking pictures of his friends and moved to Paris at age 20, though he made his career in the United States. His work as an assistantto Hans Feurer, a Swiss photographer who worked with Vogue, brought him to the attention of the magazine, and he began his relationship with the magazine even before he joined a girlfriend in New York in 1975.

He had a long creative partnership with the fashion editor Grace Coddington at both British Vogue and American Vogue. But it was his cover shot of Linda Evangelista for the September 1992 issue of Harper’s Bazaar, one eye hidden by an arm holding up an “A” of the title, that declared the arrival of a new editor, Liz Tilberis, and a new aesthetic: clean, glamorous and unforced.

Mr. Demarchelier’s rise in magazines coincided with the emergence of the supermodels and celebrity covers, and he was an integral part of creating both. He recommended Kate Moss to Calvin Klein, caught Cindy Crawford being carried on a surfboard by a dozen adoring dudes and, in 1999, helmed the 100th anniversary cover of Vogue featuring 10 of the biggest modeling names — Naomi Campbell, Claudia Schiffer, Christy Turlington, among them — in white shirts knotted at their waists and jeans, hanging out on a ladder like a gorgeous house-painting team on a lunch break.

Like Princess Diana, Madonna was attracted by his relaxed approach, and he became one of her favorite photographers, shooting her in a black leather baseball cap and vest, arms crossed and tucked under her armpits, cigarette dangling from her lips for her “Justify My Love” single in 1990. He got Janet Jackson to pose topless for “Rolling Stone,” an unnamed man’s hands clasping her breasts from behind, in an image that signaled her emergence as a independent musical power.

With a sweep of graying hair, worm eyebrows and a squinting grin, he was not above using his own Gallic charm — and a patented form of Franglais — to get a subject to do his bidding.

“No one understands anything he says,” Grace Coddington told The New York Times in 2016. “But he calls the models ‘bebe’ and says ‘fabulous’ and ‘diveeeeene,’ and he makes them feel beautiful.”

Beyond magazines, Mr. Demarchelier worked with such brands as Christian Dior, for which he also did a book, “Dior: Couture,” in 2011; Ralph Lauren; Chanel; and Giorgio Armani. He photographed the Pirelli calendar three times: in 2005 (in Brazil), 2008 (China) and 2014 (the last, for the calendar’s 50th anniversary, in conjunction with the photographer Peter Lindbergh and featuring — again — bevies of supermodels).

In 2018, as the fashion world grappled with its history of sexual harassment and abuse of power, Mr. Demarchelier was the subject of an article in The Boston Globe in which numerous models alleged unwanted advances. He denied the accusations, but his relationship with Condé Nast was terminated.

In 2007, the French Ministry of Culture named him an officier dans l’ordre des arts et des lettres, and the Council of Fashion Designers of America gave him the Founder’s Award. In 2008, he appeared as himself in the “Sex and the City” movie photographing Carrie in wedding gowns for — natch — Vogue — and the next year the Petit Palais showed a retrospective of his work entitled “The Cult of Personality.”

“I like to photograph the positive way of life,” Mr. Demarchelier told The Times in 2016. “I like the beauty, the beauty inside.”

He is survived by his wife, Mia, three sons and three grandchildren.