You know when you got arrested in that car wash in Kansas - were you going through it in a car, or on foot? "I was in a car," Mary Gauthier says. "Why?"

For some reason I thought you went in there as a pedestrian.

"No," she replies. "That sounds more like British behaviour. I was definitely in a vehicle."

But you were arrested.

"Right. I'd been discharged from a chemical dependency unit. They recommended I go to this halfway house in Salina. They gave me a job working at the car wash. It was so cold. I had to drive the cars through. They came out with the ice and road salt washed off them. I also removed any object of value from them, on the way."

How old were you?

"Seventeen. I spent my 18th birthday in jail. Charges were dropped as long as I promised never to return to the state of Kansas. My parents took me home to Louisiana. I lasted there a week. Then I ran away."

Gauthier was already five years into a drinking career that is recalled in her song "Drag Queens in Limousines":

"I hated high school; I prayed it would end/The jocks and the girls, it was their world, I didn't fit in/Mama said: 'Baby, it's the best school that money can buy/Be strong, hold your head up. Come on, Mary - try.'"

The next verse begins, in a perfectly executed deadpan tone:

"I stole Mama's car on the Sunday, and left home for good."

Mary Gauthier is not a typical product of the music business. Smart, engaging and self-possessed, you might say she has lived her life in reverse. Her indulgence in the kind of rampant misbehaviour normally engendered by fame occurred, in her case, before she had even learned to play the guitar. Gauthier (rhymes with "crochet") didn't write her first song till she was 33 - five years after she'd renounced her vices: "Mainly alcohol, cocaine and heroin."

No crack?

"Only because it wasn't around."

She has developed an extraordinary repertoire of songs that - to adjust, for gender, the phrase that Elvis Costello once used about The Band - are the work of a woman, and not a girl.

Gauthier, now 44, is the latest and most brilliant recruit to the country tradition graced by Lucinda Williams, Nanci Griffith and Steve Earle; music distinguished by its wit, intelligence and social commitment, devoid of the cloying sentimentality that makes so many people gag the moment they hear a pedal steel guitar. Her most recent album, Mercy Now, bears comparison with the work of any of the above artists, and it's fair to say that if Bob Dylan were to produce new songs of this quality, we would be talking about the unexpected artistic resurrection of the man from Hibbing. Much of her work draws on her difficult upbringing in the company of an alcoholic father, who is evoked in Mercy Now's classic song, "I Drink":

"At night he'd sit alone and smoke/I'd see his frown behind (omega) his lighter's flame/Now that same frown's in my mirror/I got my daddy's blood inside my veins/Fish swim, birds fly/Daddies yell, mamas cry/Old men sit and think... I drink."

We're talking in her townhouse in Nashville, where she's lived for the last four years, though she concedes it is not her spiritual home. Gauthier is a lesbian songwriter in a city whose men, certainly, are not famous for being in touch with their feminine side, and she has nothing in common with the dismal, formulaic FM radio output that Nashville, at its worst, has come to represent. This former Wendy's burger waitress and philosophy major is a refreshing exception to the industry rule whereby performers automatically develop an ego in direct proportion to their fame.

"I got interviewed by one writer who started with the line, 'Mary Gauthier is a woman who clearly doesn't care how she looks,'" she says. "I do too. It's just that I'm not very good at it."

Her 60,000-word road diary, posted on her website, is a welcome antidote to the self-glorifying drivel which frequently passes for autobiography in the music business. She records her experiences over the past five years, in which time - before she signed, in 2004, to the prestigious Lost Highway label - she has endured conditions that shouldn't be imposed on anybody: bug-infested B&Bs, arriving in towns with insufficient money to buy herself dinner, and flying Ryanair.

Her writing reveals her frankness, her gift for a phrase, and her modesty: Mary Gauthier's diary contains a number of observations that you can't imagine coming from the pen of some other women performers, such as Madonna. These include (on being met by a car at Malmo airport), "I hope I am worth all this fuss," and (in a Tyneside motel, where she found used condoms in the bathroom), "The coffee comes in an individual pot, so you get a good amount up front." Upgraded on a short internal US flight, she writes, "This is very exciting for me. It hardly ever happens and the comfort is a luxury I rarely get to experience."

On her first visits to London six years ago, when money was tight, she used to stay with her most loyal champion, Radio 3's Andy Kershaw.

"Mary is one of those artists whose presence immediately fills and commands a room," says Kershaw. "Success has never changed her. For all the wonderful songs she's written, she still seems genuinely bewildered that people would want to pay her any attention. That said, she is precisely the sort of person you'd want to have next to you in a bar-room brawl."

She has won over audiences in such challenging venues as a small club in Aberdeen, where "one trashed guy kept shouting, 'Show us your heart Mary!' I love those guys, I really do. It's amazing, but that is a typical Mary Gauthier audience all over the world: bikers, union guys, cowboys, working-class stiffs, a lot of them unable to make sense of troubled relationships. It's a beautiful thing, when a sober lesbian from Baton Rouge can emotionally connect with a drunk lorry driver from Aberdeen."

If conflict and anguish are prerequisites for a creative life, Mary Gauthier was blessed from the start. "Born a bastard child in New Orleans," she sings on her 2002 album, Filth and Fire, "to a woman I've never seen/I don't know if she ever held me/All I know is she let go of me."

"What I was told," she explains, "is that I was born to a mother who was a Catholic, while her boyfriend was not. They couldn't get married unless they put me up for adoption. I was taken to St Vincent de Paul's Catholic charity, where I spent the first nine months of my life."

Have you tried to contact your biological mother?

"I spent years in therapy, trying to resolve that. Finally I hired a private detective. Within a week, I had her phone number. What I decided to do was to have the detective call her and say: 'You have been found. But do you want to be?'"

Gauthier pauses.

"I told the detective to give her my number and direct her to my website, so she could see what I look like."

Months passed, and her mother never rang.

"Finally I called her. We only spoke once. She said she didn't want to be found. She cried a whole lot. I feel terrible for her. She married a man who wasn't my father. He had two daughters from a previous marriage, who she raised as her own. The man died; she'd never told him, or the girls, about me. She felt it was too late now to do that. I hoped maybe that year she would send me a birthday card, because she had my address, but she didn't. What can I do? Stalk her?"

She was adopted by Barbara and Joe Gauthier from Thibodaux, "a two-church town" 90 minutes south of Baton Rouge.

"I think there is something about being detached from your mother at so early an age that makes you a sort of spectator of life. My adopted mom made a point of telling me just how hard it was to get me. They had to go through the full frigging inquisition from the Catholics. She said: 'We had to work to get you. We wanted you.' And I believe they did. It's just they didn't know what the hell to do with me once they got me."

Her younger brother, also adopted, and also a former drug user, is serving 20 years for armed robbery. Mary, three years his senior, says she had drunk herself unconscious on sloe gin by the time she was 12.

This is behaviour you must have observed in your father.

"I stepped over him for years, passed out in the living room."

And as a child, you repeat what you see.

"I believe so. At the same time I was terribly shy. Worse than shy. Shy would have been a huge step up from where I was. At 15, I went through my first detox. I was consumed with fear. I used alcohol as a way of trying to soothe the terror."

Terror of... ?

"Other human beings."

Not, some would say, an irrational fear.

"Well you're right, there is justification for it in certain circumstances. But the alcohol just increases the fear."

What made you so anxious?

"I think my problem was everything - biology, environment, the lot. Going out into the world was very scary. Drink and drugs allowed me to venture out a little bit. I say this as a 44-year-old woman. Back then, people believed I was fearless."

On what evidence?

"I would do anything; say anything. Like, someone would give me a handful of pills, and I'd just swallow them. They'd dare me to ride a motor scooter down a crowded sidewalk; I did it. At speed. Hopping wheelies. I did these things, but I was terrified of people. Of course," she adds, "it is a form of arrogance to assume that other people are even thinking about you."

Unassuming she may be, but you do get the feeling that if you said something she felt was dumb or insulting Gauthier would most definitely let you know; a resilient independence of mind seems to have been with her from the start.

"I simply cannot abide by other people's rules. It would be fair to call me unemployable."

As a teenager, one of her first jobs was serving burgers from a drive-through window. She was reprimanded by a manager for asking a customer to wait a minute for an order.

"The next day I came back with my key to the joint, and threw it through the drive-through window. I said: 'You know what? Shove it.' It was corporate stupidity at its most beautiful."

For years, she balanced her drug habit with jobs in catering.

"I never lost the work ethic. But once I started drinking, then I'd want drugs, and it would just be an all-nighter. It was real obvious, in a John Belushi kind of a way, that either I would quit or I would die. I put myself into very dangerous territory. I was taken advantage of, by older men. I bet you couldn't find one young woman, in that sort of environment, that hasn't been."

Are you saying you were raped?

"Rape may be too big a word. Because I was drunk. And I would get in the car. And I would assume that this person (omega) would have drugs, and we would go and get high. I may piss off some feminists here, but I think I put myself into those situations. I was naive. I couldn't see where those evenings were headed."

She found a kind of stability, she says, when she was 19 and was hired by a restaurant in Baton Rouge to wash dishes.

"I was a good dishwasher. So they taught me how to cook. Which I did well. Then I became assistant manager. The owners said if I stayed a year, and ran the place at night, they'd pay for me to go to Louisiana State University. Which they did. I took philosophy classes in the day. So I was reading Descartes in the afternoon, then getting completely fucked up in the evening. If I had an exam I'd have just a couple of joints and some beers, and say to myself: 'I'm gonna study now, but when I pass this paper I will get high.' I made myself earn it. That's how I made it through college."

She quit LSU six hours short of graduating, aged 24, and moved to Boston.

"I got a job as a restaurant manager, so suddenly this alcoholic drug addict was running a piss-elegant café across the street from the Ritz Carlton, charging people $4 for a freaking éclair, with a straight face. I ran it for a couple of years - still using, still messed up. I met many people with money and I found two who agreed to put up the money for a restaurant. We agreed that I'd do all the work, have the idea, and run the place; they just supplied the capital."

On the opening night at Dixie Kitchen, she was arrested for drunk driving.

"I spent that night in jail: 13 July, 1990. I haven't had a drink or taken a drug since. That was when I realised I had to quit."

Her main addiction, by this time, was heroin.

"When I stopped I was very sick. My eyes were sunken. I remember feeling physically, spiritually and mentally exhausted. I was so tired. It seems to me, on reflection - this may be the songwriter in me speaking - that my perception that my mother had just kind of handed me off propelled me into motion. I spent years running and running. There was a momentum there, and it has taken a great deal to stop that.

The Sunday Times recently wrote of you: "A steady relationship remains beyond her."

"I had girlfriends when I was using, but I couldn't say I had steady relationships, because I wasn't a steady person myself. Now I do have a stable relationship, better than it's ever been. That is to say that I have a girlfriend now, but we don't live together. I'm not sure it would be fair on anyone to have to live with me. I need a lot of time alone with a guitar."

She began writing songs in 1995.

"I made my first CD, Dixie Kitchen, three years later. I had never played a gig. But I made it to the Boston Music Awards. I didn't win, but I was up against people who'd been on the road. That gave me confidence. All those years of destruction were receding in the rear-view mirror. A healing took place in me."

You've drawn on your dysfunctional past in songs like "Drag Queens in Limousines" and "I Drink", but you're not one of those writers who imagine, in the Dylan Thomas tradition, that they need alcohol or narcotics in order to create.

"Absolutely the opposite, because I've only ever written songs sober. Once I was sober, I noticed I had some ability to put words together. It was then that I started songwriting. At the same time, for me, the music has always definitely come from somewhere else."

Which is frightening.

"Well it is, very, because you can't be sure that it will always come. I am not a writer who hammers out a song as a commercial venture. That muse has to whisper in my ear."

Of the four albums she has produced so far, every one has been markedly better than the last. She has the ability, not given to every popular songwriter, to inhabit another character when she writes. One of her greatest songs, "Christmas in Paradise", is about a homeless man living in the shadow of the Hyatt Hotel in Key West, Florida, and opens with the line: "Davey stole a Christmas tree from Kmart last night." She wrote a wonderful anthem dedicated to Karla Faye Tucker, the heroin addict who converted to Christianity during her 14 years on death row: George W Bush achieved the distinction of being the first governor of Texas to order the execution of a woman when he rejected his fellow-believer's appeal for clemency in February 1998.

"Actually, writing about Karla Faye Tucker - OK, I haven't ever killed anybody, or been on death row - but I have been the kind of drug addict she was, so on one level I understood her almost instinctively."

Gauthier is expected to play in the UK again this summer, though her dates have not been finalised. Two songs on her next album, due for release in the autumn, will deal, among other things, with the plight of her native New Orleans in the wake of last August's floods.

"I felt Mercy Now was the best possible work that I could do," she says. "If I do better than that I will shock myself. I am scared to death of making the next one. Really."

Gauthier drives me back across town, to the Union Station Hotel in Nashville. She pulls up in front of the historic former railroad terminus, which still has the destination board, with the timetable for the Dixie Flyer, behind reception. It's not tremendously expensive, but clean and atmospheric: the kind of place she might stay in on tour these days. The pressures of life on the road have been eased, she says, but not eradicated.

Didn't you once write a diary entry about a particular bed you sleep in, I ask her, that brings on terrible dreams?

"That's at my producer's house at Lake Travis near Austin, Texas. I don't know what it is about that bed. The dreams pile up. I'm scared of it. I wake up tired."

Dreams of what?

"Dreams of the life I have lived. Emotional conflict. Loss."

No soccer at all, then?

"Not so much soccer, no. More grief, heartache and distress. The things I always go to when I write. Ex-lovers."

I have a cutting in my bag, from a British paper, that reads: "Lesbianism adds its prurient frisson." What does that mean?

She gives a mischievous look.

"I'll tell you right after I've looked up that word - prurient."

This question possibly deserves a punch in the mouth - but do you think that part of what drew you to narcotics was unease at what a largely repressive society, in the Southern States 30 years ago, might make of your sexuality?

"Well, I don't think I have ever been any other way. So I had to make peace with that at a very early age. There have been a couple of men in my life that I really enjoyed - physically, emotionally and spiritually. But I have never been head over heels in love with a guy. I have a predisposition towards women. But that doesn't mean that I don't love men. Especially if they drive a lorry and come from Aberdeen."

Gauthier laughs.

"And the reason is that we are made of the same stuff. Those are my people. Because what matters is what is in your soul. The rest is just bullshit. I used to feel that life was especially hard to me. I was wrong. Because we all struggle. And it's the struggle that bonds us. It's not the victories that bond us, it's the defeats. I believe my demons were born of a fear of humanity, based on self-hatred. Not because I am gay, but because I am not worthy."

She pauses.

"I could have been the prettiest girl in the class and the quarterback's girlfriend, and I would still have been troubled by that same feeling. I have had to learn, in my sobriety, to feel comfortable about people. It's been one hell of a ride. And," she says, beginning an observation whose truth is confirmed by the admiring expressions of the Union Station bell-boys, who have abandoned their posts to get a better view of a performer at the most exhilarating stage of her career, "I am just about there." s

'Mercy Now' is out now on Lost Highway Records