Rita Moreno was all of 6 when she made her professional debut, duetting with her Spanish dance instructor on a stage in Greenwich Village in New York. “I remember every detail,” she said.

She wore a traditional, resplendently ruffled dress. “We danced a ‘jota’ — that was a country dance. And we played castanets. My mom let me put on lipstick — I was so thrilled.” It was 1937.

For the next eight-plus dec­ades, Moreno, who will turn 90 in December, has found her way to the spotlight. And she is still dancing, as we see in the opening moments of a new documentary, “Rita Moreno: Just a Girl Who Decided to Go for It,” which shows her kicking up her strappy heels at her own Cuba-themed birthday party.

She also set up the party. “Boy, I hate doing this,” she says in the film, unwrapping silverware by the chafing dishes. “You can tell I’m not a real star, because somebody else would be doing this.

“That’s why you must never really believe anything about your fame,” she continues, with a curse. “It goes up and down.”

Moreno, who is Puerto Rican by birth and Hollywood by steely determination, occupies a singular place in the cultural firmament. She is indisputably well crowned: She had minted her EGOT status, which denotes honors in all four major arts contests, by 1977, including being the first female Latina actor to win an Oscar, for her indelible turn as Anita in “West Side Story.” The trophies haven’t stopped piling up; if there were an EGOT for lifetime achievement awards — Kennedy Center Honors, Presidential Medal of Freedom — she would have earned that, too.

Those accolades were largely for Moreno’s triple-threat talent. What has been less heralded is her depth as a path-breaker — as a person of color, as a mother (and now grandmother) and as an irrepressible (sometimes ignitable) activist and personality.

“She’s obviously an icon for all the noteworthy reasons — but she’s a kick in the pants, too,” said Rep. Jackie Speier of California, her friend of two decades.

And as Moreno’s career propels forward — she will next be seen in Steven Spielberg’s remake of “West Side Story,” which she also executive-produced — her unorthodox status only grows. There are few compatriots whose longevity stretches from before the studio era (Louis B. Mayer signed her to her first contract, calling her the “Spanish Elizabeth Taylor”) to reboots, the meme age and beyond.

For Mariem Perez Riera, the Puerto Rican filmmaker who directed the documentary, Moreno was foundational. “I’ve known about Rita since I’ve known about movies,” she said.

Still, persuading Moreno to do the documentary took nearly a year. “I just didn’t know if I wanted to entrust anyone with my life,” Moreno said. “Because if I was going to do this, I was prepared to be completely truthful.”

During the production, she added, “That’s one of the things I remember reminding myself of: Rita, don’t try to charm the camera.”

She agreed to be filmed without makeup — and even more reluctantly, without a wig. She gave the documentary team a key to her home in Berkeley, Calif., so they were there when she woke up, and followed along as she drove herself to the studio for “One Day at a Time,” the sitcom on which she starred as the scene-stealing Cuban grandmother. (Her grandson on the show was played by Perez Riera’s son, and the documentary was the brainchild of Brent Miller, a producing partner of Norman Lear, the series’ creator.)

Moreno also delved into painful topics, some of which she wrote about in her 2013 memoir, like her long, tumultuous love affair with Marlon Brando, which included a botched abortion and ended in her suicide attempt in 1961 — a transformative event for her.

Her story is shaped by how little power she had in that era, especially as a brown-skinned woman deemed a sex symbol, relegated to roles she calls “the dusky maidens,” of various ethnicities but united in their one-­dimensionality.

“I wanted to be a movie star,” she told me. “But I never imagined that it would be so hard and so painful. Never. Never.”

Early in her Hollywood life, she said, she was raped by her agent. She continued to work with him, she says in the film, “because he was the only one who was helping me, in my so-called career. That’s what’s astonishing to me, that I thought so little of myself.” It took her years, she said, and much therapy, to build her sense of worth.

Growing up, her mother, Rosa, an accomplished seamstress, made all her clothes, and then her dance costumes. They were a conjoined but complicated pair: They came to New York from Juncos, Puerto Rico, when Rita — then known as Rosita Alverio — was 5, abandoning both the girl’s father and the younger brother she doted on, Francisco. Moreno never saw him again: her first heartbreak. And she never had the courage to ask her mother why she left Francisco behind. “As strong as she was, I had a feeling that was her Achilles and that she couldn’t bear to talk about it,” Moreno said. (As an adult, she hired investigators to find him, to no avail.)

Landing in New York so early in the wave of Puerto Rican migration, Moreno, who spoke no English, was baptized by the prejudice that has trailed her throughout her life. Even Anita, whom she calls a role model, was painted — literally — wrong, in makeup the color of “mud,” Moreno said, alongside the other Puerto Rican characters in “West Side Story.” When she protested the uniformity, the makeup artist suggested she was racist, she said.

She was still being offered stereotypical parts into her 60s, she said. And even in the past few years, at a high-caliber professional occasion she would not name, she said that she experienced discrimination. “It’s something where I was just diminished, and it wasn’t even conscious on their part,” she said. “That’s what made it worse.”

“I literally went home and wept for three days,” she added. “There are scars that heal perfectly well, and there are scars that still have a very thin skin.”

Recently, Moreno herself was chastised for coming to the defense of her friend Lin-Manuel Miranda amid criticism that his film “In the Heights” suffered from colorism. “I’m simply saying, ‘Can’t you just wait awhile and leave it alone?’” she said on “Colbert.” A day later she walked back her comments.

“I was very emotional — I was so upset that my friend was being maligned,” she told me afterward. “I didn’t realize that while I was defending him, I was ignoring a very important question, and that was absolutely not deliberate.” It was, she added, “insensitive.” (Miranda also appears in, and coproduced, the documentary.)

Because Moreno seems so youthful, it’s easy to forget how much history, how many social upheavals and cultural shifts, she’s witnessed, sometimes from the front row. She was a wingspan away from Martin Luther King Jr. at the March on Washington, and advocated for rights for women and minorities long before it was “de rigueur” for stars to do so.

Moreno, who dated Elvis just to make Brando jealous (it worked), is brashly sexual. “I blush half the time when I’m with her,” said Speier, describing a fundraiser when Moreno, then around 75, performed. “She was slithering across the piano as if she was making love to it.”

Asked whether she was single, Moreno said, “You bet your ass.” She has concluded that she is too independent — and perhaps too ambitious, still (she is filming an indie comedy now, in which she plays opposite teenagers) — to be tied down.

“I love being by myself,” she said. “It’s not hard to be alone. In fact, it’s great, if you like the person you live with.”