Ellen Stewart, the founder, artistic director and de facto producer of La MaMa Experimental Theater Club, a multicultural hive of avant-garde drama and performance art in New York for almost half a century, died Thursday in Manhattan. She gave her age as 91, though biographical sources differ on her precise date of birth.

Ms. Stewart had a history of heart trouble and died after a long illness, said Sam Rudy, a spokesman for La MaMa, where she had lived for many years in an apartment above the theater, on East Fourth Street in Manhattan.

Ms. Stewart was a dress designer when she started La MaMa in a basement apartment in 1961, a woman entirely without theater experience or even much interest in the theater. But within a few years, and with an indomitable personality, she had become a theater pioneer.

Not only did she introduce unusual new work to the stage; she also helped colonize a new territory for the theater, planting a flag in the name of low-budget experimental productions on Manhattan’s Lower East Side and creating the capital of what became known as Off Off Broadway.

She was a vivid figure, often described as beautiful — a dark-skinned woman whose long hair, frequently worn in corn rows, turned silver in her later years. Her wardrobe was flamboyant, replete with bangles, bracelets and scarves. Her voice was deep, carrying an accent reminiscent of her Louisiana roots.

Few producers could match her energy, perseverance and fortitude. In the decades after World War II, her influence on American theater was comparable to that of Joseph Papp, founder of the New York Shakespeare Festival, though the two approached the stage from different wings. Mr. Papp straddled the commercial and noncommercial worlds, while Ms. Stewart’s terrain was international and decidedly noncommercial.

Her theater became a remarkable springboard for an impressive roster of promising playwrights, directors and actors who went on to accomplished careers both in mainstream entertainment and in push-the-envelope theater.

Al Pacino, Robert De Niro, Harvey Keitel, F. Murray Abraham, Olympia Dukakis, Richard Dreyfuss, Bette Midler, Diane Lane and Nick Nolte were among the actors who performed at La MaMa in its first two decades. Playwrights like Sam Shepard, Lanford Wilson, Harvey Fierstein, Maria Irene Fornes and Adrienne Rich developed early work there. So did composers like Elizabeth Swados, Philip Glass and Stephen L. Schwartz.

La MaMa directors included the visionary Robert Wilson; Tom O’Horgan, who helped create the rock musical “Hair”; Richard Foreman, who founded the imaginative Ontological Theater Company; Joseph Chaikin, who founded the Open Theater; and even Papp, before there was such a thing as the Public Theater. Meredith Monk, the composer, choreographer and director , presented her genre-bending pieces there regularly.

A few La MaMa plays, like the musical “Godspell,” moved to Broadway, and others had extended runs in commercial Off Broadway house.

“Eighty percent of what is now considered the American theater originated at La MaMa,” Mr. Fierstein said in an interview in Vanity Fair in 1993, perhaps exaggerating slightly. His play “Torch Song Trilogy” was developed there.

La MaMa became the quintessential theater on a shoestring. Salaries were minimal, ticket prices were low and profits were nonexistent. For decades, Ms. Stewart often swept the sidewalk in front of the theater herself.

But an adventurous theatergoer would be rewarded there. More than 3,000 productions of classic and postmodern drama, performance art, dance and chamber opera have been seen on La MaMa’s various stages. For Ms. Stewart, a vast number of them were leaps of faith, arising from her instinct and belief that what artists need more than any
thing else is the freedom to create without interference. She would typically appear onstage before a performance, ring a cowbell and announce La MaMa’s dedication “to the playwright and all aspects of the theater.”

During the earliest days of her theater, she supported her family of artists — her children, she called them — with the money she continued to earn designing clothes. She installed a washer and dryer in the basement for them, and many a visiting artist slept in her apartment or in the theaters themselves.

She didn’t begin directing shows herself until relatively late in her life. She often said she didn’t read plays; she read people. Her gifts, as affirmed by a MacArthur Foundation “genius” award in 1985, were intuitive and hard to pin down.

“If a script ‘beeps’ to me, I do it,” she said in an interview with The New York Times. “Audiences may hate these plays, but I believe in them. The only way I can explain my ‘beeps’ is that I’m no intellectual, but my instincts tell me automatically when a playwright has something.”

Her programming stretched far wider than the American theater. It was at La MaMa that Andrei Serban, a Romanian director transplanted to the United States, refought the Trojan War with his reinvention of Greek tragedy, “Fragments of a Trilogy: Medea, The Trojan Women, and Electra.” La MaMa became a magnet for the most adventurous European and American companies, including Peter Brook’s Paris-based group. Playing there now is “Being Harold Pinter,” a politically-charged production by the Belarus Free Theater, based in Minsk, some of whose members were arrested and others forced underground by an authoritarian regime.

La MaMa’s range of activity was kaleidoscopic and multicultural, embracing an Eskimo “Antigone,” a Korean “Hamlet” and a splashy re-creation of the golden days of Harlem’s Cotton Club, directed by Ms. Stewart herself.

Ms. Stewart was a theatrical missionary, scouting new talent abroad and planting La MaMa seeds wherever she went. She produced site-specific performances all over the world — a “Medea” created by Mr. Serban and Ms. Swados, for example, at the ruins in Baalbek, Lebanon, in 1972. Satellite La MaMa organizations sprouted from Tel Aviv to Tokyo.

With the $300,000 MacArthur grant, she bought a former monastery in Umbria, Italy, and turned it into an international theater center.

Even when her network of theaters was reduced for economic reasons, she remained the avant-garde’s ambassador to the world.

“If the play is good, then it’s good,” she said when asked about her devotion to experimental work. “If it’s bad, that does not change my way of thinking about the person involved. I may be disappointed in production values, but I’ve never been sorry about anything I put on.”

Ms. Stewart was born in Chicago and spent her childhood years there and in Alexandria, La. Published sources list the year of her birth variously as 1918, 1919 or 1920. But in an interview in 2006 in the theater journal TDR: The Drama Review, Ms. Stewart said she was born on Nov. 7, 1919.

She was never eager to speak about the part of her life before her arrival in New York, and details about it are scarce. She was married at least once and had a son, Larry Hovell, who died in 1998. Her survivors include an adopted son, Duk Hyung Yoo, who lives in South Korea, and eight grandchildren.

What is known is that she studied to be a teacher at Arkansas State College and worked as a riveter in a defense plant in Chicago during World War II. In 1950 she moved to New York with the intention of going to design school, but ended up having to support herself with a variety of jobs. At one point she was a porter and operated an elevator at Saks Fifth Avenue. According to a story she often told, on a visit to Delancey Street one Sunday, she met a fabric shop owner who encouraged her dream to become a fashion designer. He gave her fabrics to turn into dresses, and when she wore her own creations to work at Saks, she created such excitement that the store made her a designer.

Her theater career began as a good turn. Her foster brother, Frederick Lights, wanted to be a playwright but had difficulty getting his work staged. Sympathetic to him and to Paul Foster, another aspiring dramatist, she began a theater in 1962 in the basement of a tenement on East Ninth Street.

Everyone already referred to Ms. Stewart as Mama, and one of the actors suggested La MaMa as a name for her space. The theater was called Cafe La MaMa, and later La MaMa E.T.C. (for Experimental Theater Club).

At first, people were sometimes literally pulled in off the street to see the shows: Tennessee Williams’s “One Arm,” Eugene O’Neill’s “Before Breakfast,” Fernando Arrabal’s “Executioner.” Ms. Stewart would sometimes present a play — like “The Room,” by Harold Pinter — without authorization.

Neighbors initially tried to close the theater down. They thought she was running a brothel, she said in interviews. Otherwise, why would so many white men be visiting a black woman in a basement?

But the shows went on. La MaMa was one of New York’s first coffeehouse theaters and became a pillar of Off Off Broadway, which sprang up as alternative theater when Off Broadway began pursuing a more mainstream audience. As word of La MaMa spread, artists flocked to it.

Gradually, federal and foundation grants came in, giving added certification to a theater that became an important New York cultural institution.

In 1969, with the help of $25,000 from W. MacNeil Lowry and the Ford Foundation, the company moved to a former meat-packing plant at 74A East Fourth Street, where it created two 99-seat theaters and office space. Ms. Stewart lived above the theaters. In 1974 she opened the Annex, a 295-seat theater a few doors down the street in a converted television studio. It was renamed the Ellen Stewart Theater in a gala celebration in November 2009. La MaMa also has an art gallery, a six-story rehearsal and studio building nearby and an extensive archive on the history of Off Off Broadway theater.

Ms. Stewart virtually never stopped working. Despite a variety of ailments, she had been putting on about 70 new productions a year. The shows will go on. The theater said it will continue to present its schedule without interruption, and Mia Yoo, who has been co-artistic director since September 2009 will continue in that capacity.

“When I think about the fact that she is in the last part of her life, even though I’ve been there a lot of her life, I can’t bear the thought of this world without her,
” Elizabeth Swados said in the TDR article. “I can’t imagine La MaMa without her. There may be a place called La MaMa that somebody brings good avant-garde international theater to, but it will not be La MaMa. La MaMa is her.”