S.I. Newhouse Jr. in 2006. The chairman of Condé Nast since 1975, Newhouse bought and remade the New Yorker and Details magazines and revived Vanity Fair. (Evan Agostini/Getty Images North America)

S.I. Newhouse Jr., the low-profile billionaire media mogul who ran the parent company of some of the nation’s most prestigious magazines, died Oct. 1 at his home in New York. He was 89.

The death was confirmed by Mr. Newhouse’s family, but the cause was not disclosed.

The chairman of Condé Nast since 1975, “Si” Newhouse, as he was known, bought and remade the New Yorker and Details magazines and revived Vanity Fair. Other magazines in the Condé Nast stable included Vogue, Wired, Glamour, W, GQ and Self.

Before selling the Random House book publishing empire, Mr. Newhouse spotted a magazine profile about a rising young real estate mogul and was inspired to commission the first book of a future president, Donald Trump’s “The Art of the Deal.”

Mr. Newhouse brought in buzz-obsessed Britons Anna Wintour and Tina Brown as editors, who became celebrities in their own right, while abruptly firing staff members who fell from his graces. Grace Mirabella learned she was being axed as editor in chief of Vogue in June 1988 when her husband saw the news on television.

S.I. Newhouse Jr. in 2011. His company also owned Vogue, Wired, Glamour, W, GQ and Self. (Mary Altaffer/AP)

Condé Nast under Mr. Newhouse was famously extravagant, paying editors huge salaries, throwing lavish parties and rarely sticking to budgets — if budgets existed at all. The company’s expense accounts were legendary, with dresses flown from Paris to New York on the Concorde and elephants brought in for fashion shoots.

“There’s no place on Earth like this,” Vanity Fair editor Graydon Carter told New York magazine in 2009. “There’s no place where you’re given the resources you need to do what you want to do and also given complete freedom to do it.”

Condé Nast focused on glossy titles that helped set the nation’s tastes, reached millions of aspirational readers and appealed to upscale advertisers.

“Our magazines represent a certain tone and audience,” Mr. Newhouse told the New York Times in a rare interview in 1988.

He said the company that his father bought in 1959 for $5 million was following in the tradition of its founder, Conde Montrose Nast.

“It was that initial orientation of Condé Nast,” Mr. Newhouse said. “He invented the form of the specialized magazine. He didn’t want a large audience. He wanted one in which everyone counted.”

But the company has struggled in recent years with the advertising meltdown. Since 2007, it has closed Gourmet, Cookie, Modern Bride, Elegant Bride, House & Garden, Jane, Men’s Vogue, Portfolio, Domino and Golf for Women. The ambitious business magazine Portfolio shuttered in April 2009 just two years after its launch, burning through an estimated $100 million.

Forbes said in March 2009 that the downturn had sliced Mr. Newhouse’s fortune in half, but his estimated net worth of $4 billion still left him the world’s 132nd richest man.

With his brother, Donald Newhouse, Mr. Newhouse owned the Staten Island-based Advance Publications, the corporate owner of Condé Nast, daily newspapers in about 20 cities and a cable TV company. Donald Newhouse ran the less-glamorous newspaper business and a cousin, Robert Miron, ran the cable assets.

Unlike other media moguls who seemed obsessed with building a publishing empire to make money, influence opinion or bask in the spotlight, Si Newhouse seemed to have no grand plan. He rarely gave interviews, had no discernible political views and imposed few cost controls on his magazines.

Associates said Mr. Newhouse simply enjoyed the magazine business and rubbing elbows with the cultural elite.

“He loves magazines, meaning the whole and all of it, the variety of things published, the business details, the visions and actions and personalities of his editors, the problems, the problem-solving, the ink and paper,” New Yorker editor David Remnick told New York magazine in 2009.

“He likes the buzz, there’s no question,” Wintour told the Times in 2008. “If you have lunch with a celebrity or political figure, he’s thrilled to hear about it.”

A short, mild-mannered man who usually arrived at his 22nd floor office at about 5 a.m. in gray slacks and beat-up loafers, Mr. Newhouse was often described as shy and socially awkward.

His manner made for messy dismissals. Louis Gropp learned he was being fired as editor of House & Garden in 1987 while vacationing in California. Mr. ­Newhouse called and asked if he had been reading Women’s Wear Daily. When Gropp said no, Mr. Newhouse got to the point.

“There have been a lot of stories in WWD that Anna Wintour is going to become the editor of House & Garden,” he said, according to Carol Felsenthal’s 1998 book, “Citizen Newhouse: Portrait of a Media Merchant.”

“Well, is that true?” Gropp asked.

“Yes,” Mr. Newhouse replied.

Val Weaver was let go as head of Self magazine in 1988, according to a 1995 biography of Newhouse by Thomas Maier, when Mr. Newhouse knocked on her door and asked, “Would you mind if we made a change in editors in chief?”

Other editors Mr. Newhouse unceremoniously let go included Diana Vreeland from Vogue, Anthea Disney from Self, William Shawn and Robert Gottlieb from the New Yorker, and André Schiffrin from Random House imprint Pantheon.

“There are certain decisions I have to make,” Mr. Newhouse told the Times in 1989 for a story headlined, “Heads Have a History of Rolling at Newhouse.” “I don’t think there is any ideal way of handling this very sensitive area.”

Many hands were wrung when Mr. Newhouse bought the New Yorker in 1985 and forced Shawn out two years later. In 1992 he brought in Brown from Vanity Fair, transforming the idiosyncratic literary journal into a more newsy publication with shorter stories and, for the first time, photography.

Mr. Newhouse’s first marriage, to Jane Franke, ended in divorce. Their son, Wynn Newhouse, died in 2010. Survivors include his second wife, architectural historian Victoria Carrington Benedict de Ramel Newhouse; and two children from his first marriage, Samuel Newhouse and Pamela Newhouse; his brother Donald Newhouse; five grandchildren; and three great-grandchildren.

A former member of the board of the Museum of Modern Art, Mr. Newhouse had a major collection of modern art including works by Picasso, Andy Warhol and Jasper Johns. He was also a major movie buff and enjoyed theater and the opera.

Samuel Irving Newhouse Jr. was born Nov. 8, 1927, on Staten Island. His father, Sam Newhouse, bought the Staten Island Advance in 1922 and used its profits to buy more papers, eventually including the Cleveland Plain Dealer, the New Orleans Times-Picayune, the Portland Oregonian and two papers that merged to create the Newark Star-Ledger.

The elder Newhouse loved to tell how he bought Condé Nast for his wife as a 35th-anniversary present.

Mr. Newhouse attended the elite Horace Mann high school in the Bronx, where his classmates included Roy Cohn, a lifelong friend. Cohn went on to become aide to Sen. Joseph McCarthy, a New York power broker and an adviser to Trump.

Mr. Newhouse dropped out of Syracuse University in New York, then worked at his father’s newspapers. In the 1960s, he moved over to Condé Nast, a part of the family business in which his father had shown little interest, and found his niche.

— Associated Press