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She died at New York-Presbyterian/Columbia University Medical Center after a brief stay, according to a statement issued by Hearst Corp.
Gurley Brown was editor from 1965 to 1996 at Cosmopolitan, a magazine aimed at young single women, which under her hand became renowned for its provocatively posed models, frank articles and racy headlines extolling the virtues of sex.
With Gurley Brown as editor, Cosmopolitan was "the sexiest woman's magazines there was," she said in a 2004 interview with Mediabistro.
Gurley Brown was at the forefront of changing sexual mores in the United States and the modern women's liberation movement when she wrote "Sex and the Single Girl," published in 1962. The cheerful book about single life encouraged women to be independent and to have sex freely, whether or not they were married.
In the same Mediabistro interview, she said when she wrote "Sex and the Single Girl" that "nobody was talking about female sexuality."
"You were just supposed to go through with it, rearrange the spice rack in your head and think about what you were going to do tomorrow while you're having sex," she said.
Frank Bennack Jr, chief executive of the Hearst Corp, wrote in a memo to staff: "Helen was one of the world's most recognized magazine editors and book authors, and a true pioneer for women in journalism -- and beyond."
Privately held Hearst is the parent company of Cosmopolitan.
"We're very sad to report that legendary Cosmo editor Helen Gurley Brown passed away. She revolutionized the mag & empowered women worldwide," Cosmopolitan said on Twitter.
Gurley Brown still kept a pink corner office in the Hearst Tower in Manhattan, according to a recent article in The New York Times.
She was married to David Brown, producer of such Hollywood hits as "Sting", "Cocoon" and "Driving Miss Daisy." He died in 2010.
Gurley Brown also wrote "Sex and the Office," "Having It All," and "The Late Show: A Semiwild but Practical Survival Plan for Women over 50."
She told Vanity Fair in 2007 that she considered her greatest achievement to be "editing Cosmopolitan successfully so Hearst didn't have to close it down in 1965, when it was losing tons of money."
She was born in Arkansas in 1922. Her father died in an elevator accident when she was ten, and the family moved to Los Angeles. She worked in advertising before taking over as editor of Cosmopolitan in 1965.
Marlo Thomas, who played the lead character in the 1960s iconic sitcom "That Girl," said on Twitter: "RIP Helen Gurley Brown - what a trailblazer!"
A statement from New York City Mayor Michael Bloomberg said the city has "lost a pioneer who reshaped not only the entire media industry, but the nation's culture.
"She was a role model for the millions of women whose private thoughts, wonders and dreams she addressed so brilliantly in print," he said. "She was a quintessential New Yorker: never afraid to speak her mind and always full of advice. She pushed boundaries and often broke them, clearing the way for younger women to follow in her path."
"Sex and the City" author Candace Bushnell tweeted: "This really is the end of an era."
Earlier this year, she gave $30 million to Columbia University's journalism school and Stanford University's school of engineering to found an institute for media innovation in his memory.
"The David and Helen Gurley Brown Institute for Media Innovation is the final achievement of her remarkable life," said Nicholas Lemann, dean of Columbia's journalism school. "We are grieving, but we console ourselves with the knowledge that Helen's legacy will live on forever here, through the Institute."