The cause was congestive heart failure, said Geoffrey Croft, a photographer and editor who had worked with Mr. Galella on exhibitions and book projects.

For most of his life, Mr. Galella was never without a camera in his hands, as as he sought to capture the intimate, unguarded moments of famous people. He didn’t work from a studio, instead choosing the sidewalk as his canvas and becoming perhaps the country’s most notorious celebrity photographer, almost as well known as some of his subjects.

He thrived on the element of surprise, wearing disguises, bribing waiters and befriending limousine drivers who might tip him off to celebrity sightings. He once sneaked into actress Brigitte Bardot’s backyard in France to photograph her in a bikini.

“You have to be sneaky, you gotta hide,” he told Britain’s Telegraph newspaper in 2008. “As soon as you see the celeb, you jump, try to beat the cops and, soon as they pull you away, it’s too late, you’ve got the picture already!”

Besides racing along sidewalks and crouching behind bushes, Mr. Galella ran into unexpected dangers. In 1973, actor Marlon Brando punched him on a New York street, breaking his jaw and knocking out four teeth.

Mr. Galella received a $40,000 out-of-court settlement and a certain psychic revenge when Brando was hospitalized for several days because of infected cuts on his hand from the encounter. A year later, when Mr. Galella sought to photograph Brando again, he wore a football helmet with a face mask.

To many people, Mr. Galella’s livelihood was a morally dubious exercise in celebrity stalking. To others, including a growing number of curators and collectors, he was creating a form of spontaneous, pavement-level art that portrayed the human side of public figures.

“He is a viper, a parasite, a stalker, a vermin,” critic Roger Ebert wrote in the Chicago Sun-Times in 2010. “He is also, I have decided, a national treasure.”

In a career of more than 60 years, Mr. Galella photographed everyone from Elvis Presley to Lady Gaga. He never asked any of them for permission.

The two people he photographed more than any others were Taylor, a glamorous movie star at the height of her fame in the 1950s and ’60s, and Onassis, the widow of President John F. Kennedy. Mr. Galella’s efforts to get close to them landed him, at different times, in a Mexican jail and a New York courtroom.

While Taylor and her then-husband, actor Richard Burton, were filming on location in Mexico in the 1960s, Burton sent his bodyguards after Mr. Galella to rough him up and confiscate his film.

“They gave me a broken tooth and a black eye,” Mr. Galella told New Jersey’s Bergen Record in 2019. He spent part of a day in jail.

“But to me the worst was, not only was I beaten up,” he added, “but they destroyed 15 rolls of my art.”

Mr. Galella first photographed Onassis in 1967, shortly before the former first lady’s marriage to Greek shipping magnate Aristotle Onassis. She lived in Manhattan, and Mr. Galella often followed her on outings to Central Park with her children.

One day in 1971, he saw Onassis leave her apartment building alone and walk up Madison Avenue. She was dressed casually, carrying her sunglasses in her hand as the wind blew through her hair. Instead of following her on foot, Mr. Galella hailed a taxicab.

He snapped a few photos of her, “and then luck played a part,” he told the Record. “The driver was interested in Jackie, and he blew his horn. Without my even asking him. She turned, and that was the decisive moment when I got Jackie with the Mona Lisa smile.”

“Windblown Jackie” became Mr. Galella’s most celebrated photograph.

“Much of what we came to love about Jackie O we discovered through Galella’s candid images,” fashion designer Tom Ford wrote in the Telegraph in 2002. “Thirty years later, they continue to have a mystique that no formal studio portrait can match.”

Mr. Galella described his photographic pursuit of Jacqueline Onassis as an “obsession” that ultimately ended up in court. In 1969, her Secret Service detail had him arrested — charges were dropped — and Onassis once instructed an agent to “smash his camera,” which she later denied.

Mr. Galella sued Onassis for $1.3 million, alleging that she was interfering with his right to make a living. The case was dismissed, but Onassis countersued, calling Mr. Galella a menace to her and her children.

“I am an absolute prisoner in my apartment,” she said in 1971. “I live in dread fear that the absolute moment I step onto the sidewalk, that man will assault me again.”

After a 26-day trial in 1972, a judge ruled that Mr. Galella had to stay at least 50 yards away from Onassis at all times. (A later ruling reduced the distance to 25 feet.)

During the next few years, he continued to photograph Onassis and admitted that he sometimes violated the 25-foot order. She sued him again in 1982. Under the threat of a fine and a seven-year prison sentence, Mr. Galella agreed not to take pictures of her or her children again.

“I never felt guilty photographing Jackie,” he said. “It’s true that I pushed the First Amendment to the limits, perhaps. But that’s what it takes.”

He already had thousands of images of Onassis, some of which he published in books and exhibited in art galleries.

“Why did I have an obsession with Jackie?” Mr. Galella mused in “Smash His Camera,” a 2010 documentary by filmmaker Leon Gast. “I’ve analyzed it. I had no girlfriend. She was my girlfriend in a way.”

Ronald Edward Galella was born Jan. 10, 1931, in the Bronx. His Italian-born father was a cabinetmaker who built pianos and coffins. His mother was a dressmaker who, like her son, was infatuated with show business.

During the Korean War, Mr. Galella served in the Air Force as a photographer. He used the G.I. Bill to study photojournalism at the ArtCenter College of Design in Los Angeles. (It later moved to Pasadena, Calif.). After graduating in 1958, he worked as a commercial photographer for stores and began to take pictures of celebrities at movie openings.

In 1960, Italian director Federico Fellini made the film “La Dolce Vita,” which included a pesky scooter-riding photographer named Paparazzo. (The word loosely means “buzzing insect” in Italian.)

Mr. Galella eagerly took to the role as one of America’s first paparazzi. He was based in New York but traveled often to Hollywood and Europe, photographing a wide range of stars, including Cary Grant, Katharine Hepburn, Duke Ellington, Greta Garbo, Frank Sinatra, Princess Diana, Muhammad Ali and Donald Trump.

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Despite his run-ins with Onassis, Brando and actor Sean Penn — who once spat on him — Mr. Galella maintained that he had cordial relations with most of his subjects. At a White House gathering in 1979, he sneaked under a rope, in true paparazzi style, and wandered the halls until he saw actress Lauren Bacall, who said, “What are you doing here, Ron?”

His wife of 37 years, journalist Betty Lou Burke, died in 2017. Survivors include a brother.

In later years, Mr. Galella cut back on photography and criticized the intrusiveness of younger generations of celebrity photographers. His archives contain between 3 million and 4 million images, which he mined for more than 15 books, including “100 Iconic Photographs,” published last year. His work is in the collections of the Museum of Modern Art, the Tate Modern in London and other museums. In the days before his death, he was preparing an exhibit of his photography at New York’s Pierre Hotel.

Original prints of his photographs sell for thousands of dollars and are collected by many celebrities, including some who once scorned him.

“We were all in it together,” Mr. Galella told Canada’s Canwest News Service in 2010. “It’s a biosphere where everyone has their place. Without me taking their picture, they wouldn’t feel like the celebrities they are.”