The cause was cardiac arrest, said Zoe Kamitses, a longtime friend.

A colorful raconteur who said she always loved pen names and disguises, Ms. Meaker wrote children’s books as Mary James and young-adult novels as M.E. Kerr (the pseudonym mimicked the pronunciation of her surname), addressing issues including racism, sexism, mental illness, disability and homophobia.

Author and critic Anita Silvey once wrote that she was “one of the few young adult writers who can take a subject that affects teenagers’ lives, can say something important to young readers about it, and can craft what is first and foremost a good story, without preaching and without histrionics.”

Ms. Meaker also wrote a few books under her own name, including the well-received memoir “Highsmith: A Romance of the Fifties” (2003), about her two-year relationship with Patricia Highsmith, the author of psychological thrillers including “Strangers on a Train” and “The Talented Mr. Ripley,” as well as the lesbian novel “The Price of Salt.” The two women met in 1959 at a lesbian bar in Greenwich Village — Ms. Meaker said that Highsmith “looked like a combination of Prince Valiant and Rudolf Nureyev” — and lived together for six months, much of it fractious, near New Hope, Pa.

Although Highsmith was the more critically acclaimed, Ms. Meaker was by then making far more money, as she told it, writing paperback thrillers, mysteries and romances, including sharply observed novels about lesbian life in America. At the time, gay men and lesbians were considered “abnormal” and “perverse,” and many of her friends were still closeted.

“There were no magazines or newspapers about us, no clubs for us to belong to,” she recalled decades later, adding: “The churches and synagogues called us sinner, as they still do, and the law called us criminals. We had no legitimacy.”

But Ms. Meaker’s breakout novel “Spring Fire” (1952), a paperback original about lesbian romance at a college sorority, demonstrated the enormous appetite for books about lesbians, selling some 1.5 million copies and flabbergasting her publisher, the Fawcett imprint Gold Medal Books.

“They had never seen such mail,” she told the Chicago Tribune. “We suddenly realized that out there were a lot of women with these feelings who had absolutely no way to express them, deal with them, or cope.”

Written under the pseudonym Vin Packer, “Spring Fire” had a sultry cover showing two women sitting on a bed in slips, and a title that Ms. Meaker’s editor conceived to boost sales, intending to confuse readers thinking of James A. Michener’s novel “Fires of Spring.” It was not the first American paperback bestseller about lesbians — two years earlier, French writer Tereska Torrès found a large audience with her novel “Women’s Barracks” — but it kick-started the lesbian pulp genre, which continued with books by Ann Bannon, Marion Zimmer Bradley, Valerie Taylor and Ms. Meaker herself.

Over the next two decades, she published almost 20 mystery novels and thrillers under her Packer pseudonym, at times incorporating gay characters and storylines into novels about murders and kidnappings. Inspired by Edward Sagarin’s “The Homosexual in America” (1951), she also wrote nonfiction books about lesbian life, including “We Walk Alone” (1955) and “We, Too, Must Love” (1958).

The books were published under the pen name Ann Aldrich, following her editor’s suggestion that she use “a soft, all-American name, like that young man in the radio serial, Henry Aldrich.” Although some lesbian reviewers criticized Ms. Meaker, accusing her of perpetuating negative stereotypes about gay men and lesbians, and focusing too much on the Greenwich Village party scene, the books also found a sizable following.

Ms. Meaker said that she received a flood of letters from women seeking to tell their own stories, to ask how they could meet lesbians in their own cities, or to request directions to bars in case they ever made it to New York. Usually the author was quick to respond, although she warned that lesbian bars often closed so quickly that her readers would have to discover new ones for themselves.

“She read everything written on queer life, from psychoanalysis to Sappho, and there was very little she didn’t have an opinion on, and very little that would keep her from giving that opinion,” said Stephanie Foote, a literary scholar who wrote the afterword to 2006 editions of the Aldrich books.

“She was such a lightning rod in those days … but she was never apologetic about any of the choices she made in her work,” Foote added in an email. “It’s dated, sure, and sometimes problematic, but it’s also vibrant and funny and shrewd, and it shows us a gay world in the ’50s that is nothing like the shameful, repressed, furtive world we might imagine.”

Mary Jane Meaker was born in Auburn, a city in the New York state region of the Finger Lakes, on May 27, 1927. Her birth certificate listed her given name as two words, although by the time she got a passport in the 1950s, it was spelled “Marijane,” according to her friend Kamitses. At some point she began using the middle name Agnes, after an aunt.

Her father ran a company that manufactured mayonnaise (during World War II, they made dehydrated onions for soldiers’ rations), and her mother was a homemaker and neighborhood gossip who “would begin nearly every conversation the same way,” Ms. Meaker recalled: “ ‘Wait till you hear this!’

“Even today, when I’m finished with a book and sifting through ideas for a new one,” Ms. Meaker continued, “I ask myself: Is the idea a ‘wait till you hear this’?”

Ms. Meaker told NPR that after realizing that she was a lesbian, she schemed to be sent away, having read that “boarding schools were filled with perversion.” She was rewarded with an education at Stuart Hall, an Episcopal school in Staunton, Va., where she was briefly expelled for using faculty photos as a dartboard.

She later attended Vermont Junior College in Montpelier and studied English at the University of Missouri, receiving a bachelor’s degree in 1949.

After moving to New York, she submitted stories to women’s magazines including Ladies’ Home Journal, writing under pen names and promoting her work by pretending to be a literary agent, meeting with publishers and editors to talk about her pseudonymous “clients.”

She also worked as a secretary for Dick Carroll, who became her first editor at Gold Medal Books. Sitting down for drinks at the Algonquin Hotel one evening, he asked her, “What kind of story is a young girl like you burning to tell?” — leading to the publication of “Spring Fire,” albeit with a different ending than what she had originally envisioned.

Because the novel would be circulating through the mail, Carroll told her, it was open to government censorship, which meant that she could not be seen as proselytizing for homosexuality. Ms. Meaker was happy to oblige, so long as it meant getting the novel published: At the end of “Spring Fire,” one of her female protagonists has a car accident and a nervous breakdown. The other visits a doctor and decides that she was straight all along.

With encouragement from her friend Louise Fitzhugh, the author of “Harriet the Spy,” Ms. Meaker turned toward young-adult fiction in 1972, publishing the novel “Dinky Hocker Shoots Smack!,” about an overweight teenager struggling to attract her mother’s attention.

Ms. Meaker “has an ear for catching the sound of real people talking,” wrote New York Times reviewer Dale Carlson, “and a heart for finding the center of real people’s problems.”

Her later novels often featured ordinary characters in extraordinary situations, like a boy who discovers that his grandfather was a Nazi war criminal in “Gentlehands” (1978) and a 17-year-old who confronts his brother’s AIDS diagnosis in “Night Kites” (1986), one of the first young-adult novels to address the AIDS epidemic.

Ms. Meaker’s other books include “Shockproof Sydney Skate” (1972), a coming-of-age novel written under her own name; “Shoebag” (1990), which reversed the plot of Kafka’s “Metamorphosis” to tell the story of a cockroach who transforms into a boy; and “Deliver Us From Evie” (1994), a lesbian romance set in rural Missouri.

In 1993, she received the American Library Association’s Margaret A. Edwards Award for contributions to young-adult literature. “When I think of myself and what I would have like to have found in books those many years ago,” she said at the time, “I remember being depressed by all the neatly tied-up, happy-ending stories, the abundance of winners, the themes of winning, solving, finding — when around me it didn’t seem that easy. I write with a different feeling when I write for young adults. I guess I write for myself at that age.”

Ms. Meaker leaves no immediate survivors. She had lived since the early 1970s on the East End of Long Island, where she started a writers’ organization, the Ashawagh Hall Writers Workshop, which she continued leading into her 80s. By then, members of the group — including crime novelist Vincent Lardo — had published more than 20 novels.

Many of her own books were drawn from memories of her own childhood and adolescence, as she acknowledged in a memoir, “Me Me Me Me Me: Not a Novel” (1983).

“Whenever you find a little smart-mouth, tomboy kid in any of my books,” she wrote, “you have found me from long ago.”