Helen Gurley Brown, the influential editor of Cosmopolitan magazine and zesty author of the 1962 bestseller “Sex and the Single Girl,” a book that helped spur the sexual revolution by declaring that women could “have it all” — including a career, marriage and great sex — died Aug. 13 at a hospital in New York City. She was 90.

Her death, of undisclosed causes, was announced by the Hearst Corp., the owner of Cosmopolitan.

The svelte and glamorous Mrs. Brown, who in her four decades at Cosmo transformed the faltering general-interest magazine into a newsstand powerhouse with a circulation of more than 2.5 million, regarded herself as a champion of feminine power even as her Cosmo covers promoted “20 ways to please your man” and other tips to attract male attention.

“Sex and the Single Girl,” written when Mrs. Brown was 40 and married, aimed to revolutionize single women’s attitudes toward their lives. The book, published a year before Betty Friedan’s “The Feminine Mystique,” sold millions of copies and became a cultural touchstone with its message that single women didn’t need to be married to enjoy sex and didn’t need to apologize for it, either.

“I think marriage is insurance for the worst years of your life,” she wrote. “During your best years, you don’t need a husband. You do need a man of course every step of the way, and they are often cheaper emotionally and a lot more fun by the dozen.”

Her book was an outrageously bold riposte to a modern culture she thought was far too Victorian in its attitudes toward women and their right to seek pleasure and get ahead in the bargain.

She cautioned that interoffice relationships were fraught with peril but that the payoffs could be crucial for women to advance in the workplace. She warned women not to take such affairs seriously and instead see any profit — from gifts or raises — as fair compensation, considering the huge disparity in salaries between the sexes.

Reliably saucy, Mrs. Brown became a frequent guest on “The Tonight Show.” But as the years passed, many in the second-wave feminist movement, including Friedan and Gloria Steinem, said Mrs. Brown and her accent on pleasing men fostered a deeply offensive approach toward female empowerment.

“Sex and the Single Girl” and Mrs. Brown’s prominent role at Cosmo cemented in the minds of her strongest critics that she was objectifying women above all else. She had defenders as well, including novelist Judith Krantz, who once said, “There is a lot women can learn from Cosmo about living intelligently.”

Mrs. Brown, a successful advertising copywriter before she wrote “Sex and the Single Girl,” saw herself as a germane inspiration to women long after the 1960s. She championed the values of hard work and thrift and credited both for her rise from a self-described “mouseburger” to one of the most powerful women in publishing.

She said she was writing for most women, who, like herself, were not born to privilege or blessed with beauty or a college education. And it was those women she was aiming to inspire in her writing, namely by taking more responsibility for the direction of their lives.

“You can’t sleep your way to the top or even to the middle, and there is no such thing as a free lunch,” she wrote in “Sex and the Single Girl.” “You have to do it yourself, so you might as well get started.”

Helen Marie Gurley was born in Green Forest, Ark., deep in the Ozark mountains, on Feb. 18, 1922.

She often described her background as “hillbilly,” and she said her Depression-era childhood left her “mildly terrified” about the prospect of bankruptcy and failure. This fear deepened after the death of her father, a schoolteacher and state legislator, in an elevator accident in Little Rock when she was 10.

She later described her mother, Cleo, also a schoolteacher, as prone to depression. Cleo Gurley struggled to keep the family together, taking in sewing and caring for an older daughter, Mary, who had contracted polio. Cleo settled the family in Los Angeles, near the hospital where Mary received treatment.

Helen completed high school in Los Angeles and was class valedictorian, an accomplishment she attributed as overcompensation for a bad acne problem and less-than-voluptuous figure. She attended Texas State College for Women before financial reversals led her back to Los Angeles.

She took shorthand classes and held more than a dozen secretarial jobs to support her family. She said she took it personally when she was not chased around the office like the other young women. Lack of harassment, in her view, was not to be envied.

She would later write in her bestseller: “If you’re not a sex object, you’re in trouble.”

In 1948, she joined Foote Cone & Belding, an advertising agency where she became executive secretary to board chairman Don Belding.

Her boss recognized her facility with words, especially in vivacious letters to him when he was out of town, and promoted her to copywriter. Several years later, working as a copywriter and account executive at Kenyon & Eckhardt, she became one of the nation’s highest-paid advertising writers.

Meanwhile, she dated several of her bosses and a number of well-known men, including champion boxer Jack Dempsey and Ron Getty, the son of oil billionaire J. Paul Getty. In 1959, when she was 37, she married David Brown, an executive with 20th Century Fox movie studios who later produced films including “Jaws,” “The Sting” and “The Verdict.”

Brown, to whom she was married until his death in 2010, was one of her biggest fans and supporters, and he urged her to write “Sex and the Single Girl” after professing delight in reading many of her early letters to boyfriends.

Mrs. Brown often used her own long and faithful marriage as an example when doling out tips for happy relationships. Among them: “Communicate maniacally” and “always say yes to sex.”

The book brought her national prominence, and that fame was heightened in 1964 with a movie version of “Sex and the Single Girl” in which Natalie Wood played a fictitious version of Mrs. Brown.

She launched a syndicated newspaper advice column, called Woman Alone, and wrote several more books, including “Sex and the Office” (1964) and “Helen Gurley Brown’s Outrageous Opinions” (1966). The latter also became the name of her short-lived TV talk show.

In 1965, the Hearst Corp. tapped Mrs. Brown to revive Cosmopolitan’s sagging fortunes. Although she had no editing experience, she had impressed Hearst president Richard Deems with a magazine proposal that fit his own goal of transforming Cosmo from a magazine for homemakers into a publication for career women.

Deems promoted Mrs. Brown as the standard-bearer of the “working girl.” She immediately converted the somewhat turgid pages of the monthly into a must-have bible for the single female, such that a social, career-driven woman became known in common parlance as a “Cosmo girl.” Job tips and money-management advice were offered along with dating strategies.

Mrs. Brown, who had written in her 1962 book that “being smart about money is sexy,” promoted brown-bag lunches and savings accounts along with features on beauty and weight loss. She always featured an alluring woman on the cover (“I like pretty, and I like skin,” she said), and in 1972, a nude centerfold of actor Burt Reynolds rocked the magazine world.

As with her most famous book, her work at the magazine made her a target of criticism. In 1970, a group of militant feminists led by Kate Millett conducted a sit-in demonstration at Cosmopolitan’s office in New York.

As the decades passed, Mrs. Brown made no serious effort to cover AIDS or domestic violence. The magazine seldom if ever mentioned children, a reflection of Mrs. Brown’s own lack of interest in motherhood. “I didn’t want to give up the time, the love, the money,” she told USA Today in 1990.

She was a relentless proselytizer for an age-defying body and boasted about the breast implants she received at 73.

A strict diet — at one point, she gave up all sugar, alcohol and caffeine — and a daily exercise routine kept Mrs. Brown slim — some say painfully thin — and she promoted abstemious habits her whole life. Still, she told Vanity Fair in 2007, the thing about herself she least liked was “my fat tummy.”

Her personality was so woven into Cosmopolitan that it made huge news when she was forced from her position as editor in chief in 1996. She was replaced by Bonnie Fuller, the 39-year-old editor at Hearst’s Marie Claire magazine. Mrs. Brown stayed on to manage Cosmopolitan’s dozens of international editions, telling reporters there were no hard feelings.

“I’m a very smart, realistic person,” she told the New York Times in 1997. “I live in the real world. . . . Even though Cosmo is working — didn’t need to be fixed — the time comes when you can’t be 75 and edit a magazine for a 24-year-old.”

Mrs. Brown had no immediate survivors. In January, she gave a combined $30 million to Columbia and Stanford universities to create a journalism and technology institute housed at both schools.

She earlier gave her papers to Smith College, the alma mater of both Friedan and Steinem, and donated money to support the Ada Comstock Scholars program at Smith, which provides tuition for late-in-life and other nontraditional students to attend the women’s college.

“I am a feminist,” Mrs. Brown told the New York Times in 1982. “Cosmo predated the women’s movement, and I have always said my message is for the woman who loves men but who doesn’t want to live through them. ‘Sex and the Single Girl’ was controversial at the time because it said a woman could be a sexual creature and not have a wedding band on her finger.”

She added, “I sometimes think feminists don’t read what I write. I am for total equality. My relevance is that I deal with reality.”

Her hard work, modest habits and traditional marriage were often overshadowed by the saucy public image that Mrs. Brown cultivated — the Cosmo girl, her philosophy summed up by a quote she displayed in her office: “Good Girls Go to Heaven — Bad Girls Go Everywhere.”