Glenn O’Brien, an influential writer and editor who was a social fixture in the downtown Manhattan art, music and fashion world for a half-century, died of pneumonia on Friday in Manhattan. He was 70.

His wife, Gina Nanni, confirmed his death, at NYU Langone Medical Center. He had been treated for an undisclosed illness for several years and contracted pneumonia a week ago.

Over the decades, Mr. O’Brien developed a kaleidoscopic career that seemed to mirror the evolution of the downtown sensibility itself.

It began in 1971, when Andy Warhol hired him to work on — and shortly thereafter, edit — Interview, Warhol’s take on a celebrity magazine.

While Mr. Warhol famously quipped that in the future, everyone will be famous for 15 minutes, Mr. O’Brien apparently took this dictum and added a personal twist: For some 45 years, he seemed to be famous every 15 minutes, and always for something entirely different.

In addition to his work as an editor, art and music columnist, essayist, and poet, Mr. O’Brien was a television host (of the punk-era public access show “TV Party”), a stand-up comedian (once opening for David Johansen’s Buster Poindexter act), and screenwriter (his “Downtown 81” had its premiere at Cannes).

Along the way, he also made his mark as a creative director (Barneys New York), advertising copywriter (including a number of major Calvin Klein television campaigns), book editor (Madonna’s “Sex”), playwright (“Drugs,” which he wrote with Cookie Mueller), and author (the tartly opinionated advice guide “How to Be a Man: A Guide to Style and Behavior for the Modern Gentleman,” published in 2011).

If Mr. O’Brien was to be believed, he even worked briefly as an underwear model; he long claimed that his were the hairy legs and white briefs that were featured on the Warhol-designed inner sleeve of the Rolling Stones 1971 album “Sticky Fingers.”

Summarizing his multifaceted career with characteristic deadpan in 2015 in a profile in The New York Times, Mr. O’Brien said, “I guess I like to be busy.”

Even in the editorial field, where Mr. O’Brien focused most of his professional energies, his output was wildly eclectic, with his high-low instincts on full display. He was perhaps best known for his witty, acerbic “Style Guy” fashion advice column, which he began at Details magazine, then wrote for GQ from 1999 to 2015.

He was also a longtime columnist and critic for Artforum and made stops at Oui, the men’s pornographic magazine; High Times, the marijuana enthusiast’s bible; Maxim, the once-high-flying “lad” magazine, which he was hired to help reinvent in 2015; Purple, Olivier Zahm’s edgy culture magazine, as well as Rolling Stone, Allure and Harper’s Bazaar.

At High Times Mr. O’Brien carved himself into industry lore when he adopted the title “editor at large.” He had meant it as a sly reference to his tenuous legal standing overseeing a magazine that celebrated recreational drug use.

“It was a joke,” Ms. Nanni said, “a reference to the F.B.I.’s ‘Most Wanted’ posters.”

Mr. O’Brien was born on March 2, 1947, in Cleveland and raised by his mother, the former Flora Sheldon, a homemaker, and his stepfather, Donald Campbell, a telephone company executive. His father, Terrance O’Brien, had died in an automobile accident when Glenn was a toddler.

As a child, Mr. O’Brien once said that he admired the sophisticated banter on television shows like “What’s My Line,” and began to daydream of making a cosmopolitan life for himself in New York.

After graduating from St. Ignatius High School in Cleveland, he enrolled in Georgetown University, where he edited The Georgetown Journal, a student literary magazine. He also ran in a small bohemian circle on campus that included Bob Colacello, now a Vanity Fair writer.

“We were into Godard, Antonioni, Rimbaud, Verlaine and Burroughs,” Mr. Colacello told The Times in 2015. “We were part of a very tiny group, maybe six guys, who were the avant-garde at Georgetown.”

That avant-garde circle was soon to grow exponentially.

After moving to New York, Mr. O’Brien and Mr. Colacello studied film at Columbia University, and Mr. Colacello started writing reviews of underground films for The Village Voice. After his review of the Warhol film “Flesh” caught the artist’s eye, he and Mr. O’Brien — still recent college graduates — became regulars at the Factory, Warhol’s star-studded downtown Manhattan studio. Not long afterward, Mr. Warhol and his lieutenants hired them to oversee Interview.

“They thought, ‘Let’s get some nice, clean-cut college kids who aren’t amphetamine addicts and see if they can run the magazine,’ ” Mr. O’Brien told The Times.

The “college kids” were soon social arbiters, befriending rising stars of the New York art world like Richard Prince and Jean-Michel Basquiat, as well as luminaries of the emerging punk rock scene, including Deborah Harry and Chris Stein of Blondie.

That social network provided the star power for “TV Party,” a cult satire of a late-night talk show that Mr. O’Brien started in 1978 with Mr. Stein. The equivalent of “The Tonight Show” for the East Village club crowd, it featured pointed political satire, open marijuana use and dozens of boundary-pushing guests, including David Byrne, George Clinton, Robert Mapplethorpe and Mick Jones of the Clash.

His TV stint ended in 1982, but Mr. O’Brien never slowed down, even in recent years, as he struggled with illness while living in an art-filled NoHo loft with Ms. Nanni and the couple’s son, Oscar, now 16. (Mr. O’Brien also has an adult son from a previous marriage, Terence O’Brien Pincus.)

Following a rancorous split with GQ in 2015, Mr. O’Brien jumped back into publishing for a brief tenure at Maxim as — what else?— “editor at large” for the new owner, Sardar Biglari.

He continued to write, including artist monographs and poetry. A collection of his poems, titled “Ruins With a View,” is soon to be published, Ms. Nanni said. “Like Art: Glenn O’Brien on Advertising” will be published in May.

Mr. O’Brien never lost his taste for the small screen. In 2015, he started a new show, “Tea at the Beatrice,” a far more genteel talk show, featuring the likes of the photographer Nan Goldin and the supermodel Gisele Bündchen on Apple TV’s M2M channel. He described it as a “more bohemian ‘Charlie Rose.’ ”

By then Mr. O’Brien was no longer the world-weary young punk with the buzz haircut and the razor-sharp cheekbones. He looked more like a godfather of cool in elegantly tailored suits, but he did not seem to believe that the decades had eroded his ability to stand out on camera.

“I think my dark hair looked good in black and white,” he said, referring to “TV Party,” “but in color, the white hair is working.”