Janet Flanner, 86, the "Genet" of The New Yorker magazine and for 50 years the author of its Letters from Paris, died at Lenox Hill Hospital in New York City Monday following an apparent heart attack.

The Paris letters began appearing in 1925, the year The New Yorker was founded. They were a regular feature of the magazine until 1970, and appeared intermittently thereafter until 1975. Miss Flanner also wrote a novel and several other books. Three volumes of her "letters" have been published in book form.

For their knowledge, wit, and understanding, the letters have been widely acclaimed as some of the finest foreign correspondence ever written.

"She's our oldest living monument," William Shawn, editor of The New Yorker, said in 1972. "As a social historian there isn't anybody like Janet Flanner . . . No one else over the decades has consistently combined that kind of literary fire and dazzling style with just good, solid, dependable reporting."

Newsweek magazine described her in 1971 as "the most lively, perceptive and authoritative chronicler of France during the Third, Fourth and Fifth Republics."

In a review of "Paris Journal, 1944-1965," a collection of her articles covering those years, Benjamin C. Bradlee, executive editor of The Washington Post and once a correspondent in Paris, wrote: "No American has watched the Poetry and politics of France since World War II with more sensuous insight and tolerant love than Janet Flanner."

Miss Flanner once said she had invented a formula for her letters "which dealt not with political news itself, but with the effect political news had on private lives." In fact, she wrote about more than politics. She was one of the American expatriates who settled in Paris in the early 1920s, and she knew almost everyone: Picasso, Gertrude Stein and Alice B. Toklas, Ernest Hemingway, Georges Braque, F. Scott Fitzgerald, Edith Piaf, Colette, Josephine Baker, Andre Malraux - the list is seemingly endless.

She wrote about the arts and artists as well as about politics and politicians. Among her books were profiles of Braque and Picasso. But she also described people whose lives otherwise were not widely remarked, or not widely remarked outside of France. In a 1939 "letter" she wrote:

"Anatole Deibler, grand high executioner of France, recently fell dead of a heart attack in a Metro station while on route to his 401st guillotining. He was 76 years old, dapper, quiet-spoken, looked like Pioncare but was handsomer, liked to go fishing alone and was the last to bear the name of his family, which was of Bavarian origin and provided France with headsmen for 110 year . . . Deibler's father, Louis, who was the last guillotiner to wear a top hat when performing his duties, married the exotic Zoe, daughter of Rasseneux, chief of another famous headsman family. Diebler leaves no male heir (as serious a lapse to a decapitator as to a king, both professions being dynastic). Deibler's only son was accidentally killed as a child by a careless drug clerk who made up a poisonous prescription."

Miss Flanner herself was short, silver-haired, and possessed flashing brown eyes. She smoked almost incessantly. It was said that she talked almost as well as she wrote. An admirer once described her face as "the Indian head on the buffalo nickel."

Her New Yorker articles always appeared under the name "Genet." She said the nickname had been bestowed upon her by Harold Ross, the founder and first editor of the magazine and a man whom she greatly admired.

"Owing to Ross' speaking no printable French he did not know that genet was the broom flower, a civet cat, and also a jennet, which is a small Spanish horse, as well as a not very reliable French journalist who after the French Revolution was the first Franco-American gazeteer," she said.

Janet Flanner was born in Indianapolis on March 13, 1885. Her father was the founder of Flanner House, a settlement house for blacks. Among the visitors there was Booker T. Washington, and Miss Flanner remembers sitting on his knee when she was 7.

"He put his arm around me and said, 'You're not afraid of me because I'm very, very black?'" she recalled in 1972. "I said, 'Why should I be. It makes no difference.' I was already a pretty educated child. Mr. Washington was no Harry Belafonte. He was quite homely, but he had such gentleness and sweetness in his face, in his actions and in his manners."

With her family, she spent a year in Germany. In 1912, she entered the University of Chicago. She remained there two years, studying English composition under Robert M. Lovett, later an editor of the New Republic and a government official. She said she was asked to leave her dormitory at Chicago because she was a "disruptive influence."

In 1916, she got her first writing job - movie critic on the Indianapolis Star. She also worked at a reform school for girls in Pennsylvania.

"It was one of the most charming places I'd ever seen," she said years later. "In June, it was overpowered with red roses. There were cottages for the girls, a little main street, a beautiful, old red brick mansion, a schoolhouse, a village store, and no walls, no gates. The girls sneaked out at night."

She went on a tour of Greece and Turkey after World War I. On the way home, she stopped in Paris. The year was 1922. Except during World War II, Paris remained her home for more than 50 years. For most of that time she lived in the Hotel Continental overlooking the Tuileries Gardens. She spent her last years there in the Ritz Hotel. She had lived in New York since 1975.

Miss Flanner began her letters to the New Yorkers at the suggestion of Jane Grant, an old friend and Harold Ross' wife. The last of them appeared in 1975, the year of her final stay in Paris. Her last piece for The New Yorker appeared in the same year - a review of collection of letters by Alice B. Toklas.

The French government awarded Miss Flanner the Legion of Honor in 1947. She received an honorary degree from Smith College and, in 1966, won the National Book Award for "Paris Journal, 1944-1965." Miss Flanner listed her interests as conversation, croquet, card games, chess and the harmonica.

Her survivors include a sister, Hildegard Flanner, a poet whose work has appeared in The New Yorker, of Calistoga, Calif.