One night in December 2004, a few hundred hipsters and art folk converged on St. Ann’s Warehouse, a progressive arts institution on the Brooklyn waterfront. The occasion was a 25th-anniversary benefit for the venue’s Arts at St. Ann’s program; the evening’s entertainment was Fire at Keaton’s Bar & Grill, an off-kilter song cycle by saxophonist and Jazz Passengers cofounder Roy Nathanson. Among the featured vocalists were rock iconoclast Lou Reed, Blondie heartthrob Deborah Harry, multimedia scribe Laurie Anderson, Chocolate Genius auteur Mark Anthony Thompson–and the fair-haired Nellie McKay.

McKay (rhymes with “hi,” not “hey”) sang the suite’s “Bar Stool Paradise” as a duet with Terrell Lee Porter, corkscrewing gaily through an Annie Ross-style vocalese. She clasped her hands together during an alto solo by Nathanson, who had the mien of a crazy uncle, and then scatted obligingly over a mid-tempo Hammond organ groove. According to the program notes, she was playing Cookie, a denizen of the fictive Keaton’s. It was difficult to know how well she was filling this role, but she seemed to have mastered another one: that of a nightclub jazz singer.

The full measure of McKay’s talent wasn’t in evidence until later in the program, when she sat at the piano to perform “Ding Dong,” one of her many smart and unpredictable original tunes. Conspicuous by her self-accompaniment, McKay brought humor, pathos and a kind of existential angst to the performance; her composure was almost as striking as the song itself, which began with the blunt and uneasily funny phrase “My cat died.” It was the briefest taste of McKay’s oeuvre, but enough to win over the house. Backstage after the show, members of the band went out of their way to compliment the singer-songwriter; one saxophonist sized her up and spread his arms querulously, exclaiming: “Where does that music come from?” Dan Zanes, an indie-rocker turned children’s folksinger who had played the arsonist villain of Keaton’s, was subtler in his praise. “I never heard her before tonight,” he said, slowly nodding his tousled head of hair. “But count me among the converted.”

By this time, McKay had shed her red cocktail dress and slipped into slacks and a sweater. The green room was crawling with fascinating people, but she was anxious to escape–across the Brooklyn Bridge, into Manhattan and up to Harlem, where she lives in a first-floor studio apartment. She was tired. “I’m getting ready to do a reading for a Broadway play later this week, and we’ve been working for the past two weeks,” she said breathlessly. Then, incredulously: “I’ve turned into a morning person.” The following night, she was scheduled to play another benefit, hosted by Wyclef Jean and featuring Roberta Flack, the rapper Common and the actress Susan Sarandon.

This was just one such scene of many for McKay, but it seemed casually emblematic. The cabaret wunderkind has been nothing if not industrious since her early-2004 Get Away From Me (Columbia), throwing her weight behind a long parade of causes. During all that time, she has been impossible to pin down to a single genre, jazz or otherwise. And yet plenty of jazz folk have turned an ear. They’re more than half right to sense a kindred spirit in McKay–but then, so is Lou Reed, who had her open for him at a JVC Jazz Festival concert at Carnegie Hall.

Now, just over a year after that night in Brooklyn, McKay has a new album, an impending Broadway debut and a feature film project in the works. In many ways, the bright young singer seems positioned for a foothold in popular culture. But is she still, at heart, a jazz singer? That depends on your definition of the term.

Certain elements of McKay’s story should ring a familiar chord for the jazz public. Like another young chanteuse, Jane Monheit, she grew up worshiping the singers who embodied jazz’s mid-century dalliance with pop. And like another genre-spanning singer-songwriter, Norah Jones, she dropped out of music school to play low-rent gigs in Lower Manhattan clubs. Along with both Monheit and Jones, McKay has professed an overwhelming affinity for the music and culture of yesteryear.

But McKay’s anachronism is at once more severe and more selective than her counterparts. She spent her childhood emulating Doris Day, Peggy Lee and Dinah Shore and even now converses with the coyly stammering decorum of a Hitchcock heroine. But she’s also a dedicated 21st-century feminist and animal-rights activist who curses casually on and offstage, especially when she’s flustered or contemplating social injustice, which is often.

Get Away From Me–a play on Jones’ smash Come Away With Me, with strident misanthropy standing in for lilting melancholia–is an album loaded with references to the likes of George W. Bush, Dr. Phil and the Olsen twins. The songs that sound most like smoky jazz standards–“Manhattan Avenue,” “I Wanna Get Married,” “Won’t U Please B Nice” and “Really”–show a rare command of old-fashioned songcraft but also a willfully sardonic streak. The songs that don’t sound like smoky jazz standards are junkyard-scrap assemblages of new wave, faux reggae and power-chord rock. She even raps on more than one delirious track.

Tracing McKay’s background can be an equally slippery business. She was born Eleanora Marie McKay in London and raised in the U.S. by her mother, a former actress named Robin Pappas. They lived in Harlem until the preadolescent McKay was mugged (a tale told with poetic lightness in “Manhattan Avenue”) and their tenant’s-rights activist was brutally murdered (an incident that fuels the fiery indignation in her tougher songs). They moved to Olympia, Wash., briefly, and then Swiftwater, Penn., more or less permanently. McKay played saxophone in her high school concert band, piano in the jazz ensemble, mallet percussion in marching band and cello in the Pocono Youth Orchestra. She returned to Harlem in 2000 to matriculate at the Manhattan School of Music. She was 16 at the time, according to her official bio, although that part of the story has been a subject of debate.

McKay maintains her birth date as December 13, 1984. Press materials at the time of Get Away From Me played up a precocious-teenager angle that was dutifully adopted by all of its early reviewers. Then came a miniature scandal in September 2004, when McKay’s estranged father, a British filmmaker, claimed she was born on April 13, 1982–which would have made her 22 instead of 19 (and an Aries rather than a Sagittarius, although that was the lesser charge). A newspaper in Buffalo ran the story big, but elsewhere it came and went. On the day that it broke, McKay called her harried publicist to make light of the flap: “Oh, and by the way–I don’t write any of my own tunes, either.”

When I met McKay for our first formal interview, on a humid night in June 2005, she was less deadpan but still slightly inscrutable. At her request, we were at the Dublin House, an endearingly shabby Upper West Side pub that had a Yankees game on one television and a Mets game on another. “This is one of the few places I used to be able to come to as a kid, with my mom” said McKay before ordering a Diet Coke. She was wearing her summer uniform of vintage blouse and overalls. One of our first topics of conversation was the fallacy of autobiography–an all-too-common misperception that the songwriter’s “I” reflects a genuine first-person voice. McKay restated her admiration for Bob Dylan, master of the protean ego, and confessed that she had at one point considered the stage name of Eleanor Sphinx, a combination of her real first name and a mythological totem. “The sphinx,” she explained, “is the eternal outsider of society–always female, asking the questions, on the outside looking in.” After a moment, she added: “I really do believe in reinvention. I think everyone should have that power.”

McKay doesn’t exactly reinvent herself on Pretty Little Head (Columbia), her long-awaited second album, but she does make disarming gestures toward a purer kind of pop. This is most glaringly obvious on a duet with Cyndi Lauper, for whom McKay opened in concert two years ago. “Beecharmer,” which the singers wrote together, bears as many of Lauper’s fingerprints as McKay’s, especially on a big-flourish chorus. (The verses, with lines like “I’m the queen of QVC,” are feasible coming from either one of them.)

The album’s other touted duet, “We Had It Right” with k.d. lang, treads more familiar territory, with a “Don’t Worry, Be Happy” bounce that flatters both singers’ styles. It sounds less labored than the Lauper track and more like a breezy confection. But both celebrity cameos feel equally sincere; McKay has made no secret of her attraction to strong and unconventional female artists. Even the title of the album underscores this point, bringing to mind a debut by another rocker McKay once supported on tour: Alanis Morissette’s Jagged Little Pill.

The tracks for Pretty Little Head were assembled over a period of months, much in the manner of pop. But this was mostly a matter of means: McKay put her own money up front, self-producing the record with her mother. The process was markedly different on her first album, which was produced by former Beatles engineer Geoff Emerick. McKay speaks highly of Emerick, but it’s clear that she relishes independence and has a strong vision of her own. “I think this record has more heart,” she told me while she was still in the process of recording. “It’s more raw, more acoustic; the first one was a little overmixed. We were all trying our best, but we were under the gun. Now, it’s more of a band sound.”

McKay has crossed paths casually with a motley variety of bands. Perhaps her strangest hookup was at the 2005 Jammy Awards, when she was matched with crunchy guitarist Keller Williams and hip-hop drummer Ahmir “?uestlove” Thompson. (She played Dylan’s “Rainy Day Women #12 and 35” and more or less tanked, although the crowd enjoyed the refrain about everybody getting stoned.) At the other end of the spectrum was a benefit concert at Carnegie Hall in February 2005, on which she borrowed an ace band from punk poetess Patti Smith. (“I was struck by her incredible poise,” guitarist Lenny Kaye later recalled of McKay.) This past fall, she formed her own working group consisting of jazz musicians like bassist Lonnie Plaxico and saxophonist Bob Reynolds. “I think it’s like having a posse,” she said before this band went on tour. “The more people you have up there on stage, the more power you have. You’re less vulnerable.”

For some of McKay’s admirers, less vulnerability may not be purely a good thing. Part of what makes the young singer such an arresting performer is the hard glint of her creative edge–especially when she’s unaccompanied, in the setting that best exposes her idiosyncrasies. A veteran of New York’s gay nightclub circuit and, more casually, its stand-up comedy scene, McKay has mastered the art of toying with expectations. Her piano playing can seem fumbling and amateurish or surefooted and fluid; her persona ranges from cool professionalism to a sort of disheveled madness. At Central Park’s open-air Delacorte Theater in August 2004, she was garrulous and effervescent–a bubbling fount of personality in a peasant blouse and those overalls. One month later at a crowded bar in Williamsburg, Brooklyn, she took her station at a Yamaha keyboard with detached decorum, wearing a storm-gray dress and elaborate upswept coiffure. She looked a bit like Sarah Michelle Gellar’s Buffy the Vampire Slayer in an especially grim tableau; when she sang the words “I was a pageant gone bad,” from “The Dog Song,” they had an unusual resonance. “I know you’re all young punks, but go vote,” she said acidly at one point, between songs. “Take time out from sleeping around, or whatever it is you do.”

After the show, McKay lingered on the sidewalk outside the club with Robin Pappas, who doubles as her manager. “Did you think it was a little heavy on the hostility?” Pappas asked, pointedly.

McKay responded with a feint. “There’s got to be a better way to make money than this. There’s an easier way to make money than performing.”

Pappas took a long drag of her cigarette and fixed her gaze on an onlooker. “Please tell me it’s a phase,” she mock-implored. Less than a week later, McKay performed at the Great Wall of China with Lauper, Boyz II Men and Alicia Keys.

I just want to write songs that you can listen to more than once,” said McKay at the Dublin House. She was responding to, if not quite answering, a question about the shifting role of protest in her music. McKay professes an open admiration for songs that take aim at an issue, like Dylan’s “Hurricane,” Billie Holiday’s rendition of “Strange Fruit,” Nina Simone’s “Mississippi Goddam” and John Lennon and Yoko Ono’s “Woman Is the Nigger of the World,” which she has proudly performed. Pretty Little Head includes a cleverly campy piece about gay marriage (“Cupcake”) and a broadside about animal testing at her neighborhood Ivy League university (“Columbia Is Bleeding”). During the months leading up to the 2004 presidential election, she spiked her performances with songs about John and Teresa Heinz Kerry, although she also took pains, in lyrics and between-song banter, to stump for Ralph Nader.

McKay is never more animated than when discussing political or social issues. Pretty Little Head takes its name from a patronizing piece of advice–“Don’t worry your pretty little head about it”–that the diminutive singer has surely heard many times. “I get so angry,” she marveled, in the middle of a passionate condemnation of right-wing chicanery. “I just want to go out with my black magic marker and deface everything.”

The scrawl of that marker is evident in McKay’s more heavy-handed material, like “The Big One,” from Pretty Little Head–songs that swing wildly at the world, addressing ills as complex and sprawling as corporate conglomeration. She uses a finer point and more delicate strokes on many other songs, which range from satirical to self-searching to coolly scathing. Whatever the metaphorical implement–Sharpie or No. 2 pencil–the handwriting remains the same. McKay uses a range of songwriting techniques and an even wider array of styles, but she’s always herself: pugnacious, ambitious, irreverent and clever.

McKay brings that cocktail of characteristics even to her perception of jazz. “Jazz–like rock & roll, like hip-hop–is a fundamentally rebellious form of music,” she said in June. “It came out of a great need to express it.” Asked whether there were jazz standards she’d consider interpreting, the singer paused for a pregnant seven seconds. “None that haven’t been done better before,” she finally replied, before citing a few favorites, including “I Cover the Waterfront,” “The Very Thought of You” and a song about New Orleans whose name she couldn’t remember.

A month later, she dropped me a line out of the blue to say that the song title was “Do You Know What It Means to Miss New Orleans?” McKay has since added it to her repertoire, in the wake of Hurricane Katrina. “It has a bazillion triple poignancy now,” she said recently, using the same breath to point out the brutal inequities that the hurricane finally exposed.

It’s a stroke of good fortune that Nellie McKay’s worldview has led her to a pair of high-profile extracurricular assignments, both of which seem tailored to her aesthetic ideals. The first is a movie based on Katherine Arnoldi’s acclaimed graphic novel The Amazing True Story of a Teenage Single Mom. This was a result of some benevolent timing: When Get Away was released, a copy landed on the desk of Broadway director Scott Elliott, who was immediately intrigued. Elliott was thinking about creating a musical film adaptation of Arnoldi’s book, and McKay’s blend of worldliness and youthfulness seemed a perfect fit. He and his partner on the project, the Hollywood producer and longtime Steven Spielberg associate Kathleen Kennedy, brought McKay aboard to write and perform music for the film. During our conversation at the Dublin House, McKay pulled a copy of the hardcover comic from her tote bag, handling it with care. It’s not hard to see what draws her to Arnoldi’s tale, which begins with the trauma of rape, deals with workers’ rights and ends on a note of clear-eyed hope and self-reliance. A few of the most striking songs on Pretty Little Head–“The Down Low,” “There You Are in Me” and “Mama and Me”–bear some connection to the project.

Not long after Elliott started working with McKay, he began to consider her for another major endeavor: a Broadway revival of Bertolt Brecht and Kurt Weill’s Threepenny Opera, based on a new translation by the actor Wallace Shawn. “I just had this strange feeling that she could act,” Elliott said, “because she is so gifted and her work is so truthful. I was just so taken with her. So I mentioned this Threepenny thing to her, and said that she would have to come in and audition, all the while thinking: ‘It would be incredible if she could make it happen.’ But I didn’t know. I gave her some coaching, and then she came in and auditioned for me and the producers, and basically just blew everybody away.”

When the Roundabout Theatre Company’s Threepenny Opera opens on Broadway in April, McKay will play Polly Peachum, a leading role. Her costars will be the Tony-winning actor Alan Cumming as the dapper scoundrel Macheath and Edie Falco, of The Sopranos, as Ginny Jenny.

Polly Peachum is at once an ingenue; before her first appearance onstage, her father characterizes her as “nothing but a mass of sensuality.” But over the course of Brecht’s hardnosed play–in which Polly defies her condescending parents, marries Mack the Knife, discovers Jenny, the prostitute wife he already has, and takes over his criminal operation–the heroine proves herself a steely operator. At one point, after Polly has heartily sung the vengeful shanty “Pirate Jenny,” a somewhat shaken Macheath advises her in Brecht’s text: “I don’t approve of your doing this play-acting, kindly drop it in the future.”

The young Miss Peachum slips into her various roles without hesitation, demonstrating either guilelessness or savvy, or perhaps a touch of both. In this regard, Polly and Nellie seem one and the same. But that doesn’t diminish the challenge of the role–and by all accounts so far, McKay has surmounted that challenge with uncommon success.

“It’s shocking, really, for somebody so young and inexperienced, as far as acting, to be so in touch,” raved Elliott. “It’s astonishing. One of the things that I’m gleefully waiting for is to see what people are going to say about her. I think that she’s going to take over Broadway, basically. She’s the freshest thing that anybody’s seen in years. And I’m not alone. Everybody who saw the workshop–from my producers to Wally Shawn–we couldn’t imagine anybody else playing the part, ever. There was something oddly definitive about her understanding of the character.”

Whether audiences end up agreeing, it’s clear that, as with Teenage Mom, the themes of Threepenny Opera energize McKay as much as her character does. The falsehood of so-called reputable society is not a foreign idea to her; it’s one of the most common underpinnings in her oeuvre. The curious thing is that McKay, while working on these projects, has also produced what she terms “a fair shambling of narcissistic love songs” on Pretty Little Head. It’s as if she felt compelled to counteract the darkness of Brecht and Weill and Arnoldi’s worlds with doses of sweetness and light. One of the most affecting pieces on the album seems to hold both sides of the picture in a quiet embrace: “Gladd” is a heartbreaking tribute to Glad Patterson, an activist and Greenwich Village habitue who died in 2003. A former assistant to the actor Rex Harrison, Patterson was one of McKay’s early hecklers and supporters as well as a conspicuous role model.

There are more than a few roles in McKay’s repertoire these days, of course. At the Dublin House, she mused on this topic while discussing Threepenny: “It’s very nice to play a part and actually be playing a part. People still confuse you with that part; I don’t think you can ever get away from that. But it’s nice to interpret as opposed to write your own thing.”

She paused thoughtfully. “And yet even in that scheme, you feel your loss of power. I think other actors can back me up on this; you’re not in a powerful position. Writing, you’re in a much more…”–she trailed off, and then snapped back–“I like having power.”

Listening Pleasures:

Paul Simon, Graceland (Warner Bros). “Great mongrel music,” McKay says.

Bob Dylan, Masked and Anonymous (Sony) soundtrack. “All over the place, beautifully.”

Gearbox:

“Big old funky piano and a cardboard box,” says McKay.