At the start of the decade, Mariah Carey's star looked on the wane - bad record sales and rumours of a breakdown plagued the R&B diva. But, with her latest number-one album and a string of multi-million dollar deals, the queen of excess is ready to reclaim her throne.

'I haven't been out of the apartment for a few days,' Mariah Carey tells me. She's pretty much just woken up. It's 8:30pm on a Friday night, and she's been nocturnal all week. I follow her on a tour of her home - she totters down the hall in a pair of black Agent Provocateur Hollywood Pom Pom mules. Carey says she always wears heels around the house because she walks on her tippy-toes. And, anyway, she says, 'They're not high compared to a street shoe that I would wear, darling!'

Mariah Carey's house is at the top of a tower, a duplex penthouse in TriBeCa, an area of town that has just been named the most expensive in New York City. Her living room on the 5,000sq-ft 17th floor is warm and welcoming, with cushions and flowers - everything is taupe and beige and gold and cream. Marilyn Monroe's white piano, for which Carey paid over £350,000 in October 1999, is surrounded by dainty white ropes. A local soul radio station plays on hidden speakers.

Carey's housekeeper helpfully describes Mariah's outfit as 'around-the-house casual': in addition to the heels, she's wearing black tailored Juicy Couture pants and a tank top, the combination making an offering of her abundant chest. Honestly, the 36-year-old Carey looks stunning and young.

In the large kitchen, Jack, the little terrier, clicks-clacks around the room clad in a glittery collar. Carey's been debating the 'flyness' of his accessory - was it too-too? 'No one's allowed to give him treats but me,' she begins telling me, then her mobile rings. (Her ringtone is Bow Wow's Fresh Amiziz.) She talks for a few seconds, tosses a ball for the dog, then leads me upstairs, to the penthouse's own small penthouse, and out onto a large terrace. Carey wants to turn this space into Club Mimi: fresh air for her and her friends - not like all those crowded and sometimes dangerously smoky joints she says she always ends up in; those inevitable late-night trips to Manhattan's prime celeb hangout, Bungalow 8.

I think we're just about ready for the interview when Jack decides to pee in the corner of the terrace before careering off into a Moroccan-themed room and then off downstairs. 'If he's running around down there he'll attack my drapes,' worries Carey, before hitting the intercom and saying, 'Can you guys grab Jack, please? He's having the time of his life.'

Finally, happy that Jack no longer poses a threat to her curtains, Mariah, surrounded by a hundred plump cushions, kicks off her heels, and snuggles up on the Moroccan couch. She speaks in the low voice she's had all her life. 'The other kids at school would say, "Could you call and pretend to be my mom?'" she recalls. Her barest whisper fills a room in a very strange way, and when she laughs, she laughs a lot. She can, and does, break into flawless song mid-sentence.

This month sees the release of Say Somethin', the latest shiny slice of perfect popular R&B hewn from the rich seam that is Carey's latest album, The Emancipation of Mimi. Written with Pharrell Williams, Carey joins Snoop Dogg on a deceptively infectious musical trip that's guaranteed to be one of the anthems of the summer.

Carey says she hears song bits in her head all the time - fleeting bits of melisma and complicated modulations and melodies - but so briefly that she sometimes has to call her answer phone to record them or have a musician around to help her explore them. She prefers working late at night, in a dark studio. Recently, she's been recording in Anguilla in the West Indies and on the island of Capri, looking out over the Mediterranean. 'I really work well by the water,' she says. And where do these musical ideas come from? 'I don't know where,' she answers. Then, 'No! Obviously, I do - those moments of creative bursts are gifts from God,' she smiles. Over the course of the interview, the only other singers Carey mentions are two much lesser-known gospel masters: Kim Burrell and Karen Clark- Sheard. 'They're just really channeling - it's coming from God. A lot of people, it's too much for them - they can't follow it. It's just too deep for them.'

Carey, whose career nearly stalled and died, has had her revenge, but it's not the one you might think. 'Depending on the designer,' she says, covering herself up in a moment of reflex physical discomfort, 'I'm between a size four and a six [a UK8 and 10].' Then she boldly shows me her flat stomach. She could, she tells me, get down to a size two if she really wanted to, but she won't. She's thin now because it really made her mad when people talked about her being fat. 'I had to change my relationship with food,' she says. 'I know - it sucks.'

'I have a trainer,' Carey continues, 'but she lives on St. Barts, in the West Indies. Of course, only I would have a trainer who lives on St. Barts - but she's fabulous. She weighs one pound soaking wet,' she laughs. 'She is a dictator!' They do water aerobics. "'Encore" is her favourite word.'

But The Emancipation of Mimi and its absurdly huge success after a few difficult years when Carey's singles and albums underperformed was also powered by something else.

'People ask me, "What do you say to all those naysayers who wrote you off?" And I don't have to say anything to them.' She believes the album was a success because she followed her heart and her ear, and she thinks her emotional peace with all her mess of a history shines through. The subtext of the way she talks about her older work is that producers and executives wanted her to make mainstream music; they thought audiences wouldn't get her being herself. 'Or so the record company thought. But I guess they were wrong,' she smiles.

A quick biography. Mariah Carey was the woman who sold the most albums in the Nineties: her first came out in 1990, and her 1993 album, Music Box, has sold tens of millions of copies. In 1998, she divorced her star-making, music-producer husband, Tommy Mottola; they married in 1993 when she was 23 and he was 20 years her senior. In 2001, at the top of her game, she signed a reported £55 million, five-record deal with Virgin.

Her movie, Glitter, was released in September 2001, but had little success at the box office. At nearly the same time, she was hospitalised in Los Angeles, California. The press called it a nervous breakdown; Mariah described herself as suffering from exhaustion. In 2002, she left Virgin by mutual consent, but soon found a new home at Island DefJam. Her comeback was slow: it began with a stellar rendition of the US national anthem at the Super Bowl in February 2002. In late 2002, her poppy album, Charmbracelet was moderately successful. Then came the Grammy grabbing The Emancipation of Mimi.

Carey flexes her fingers, her jewelled butterfly ring flapping. She's comfortable enough with her fame, but not with the way fame works these days. 'People's careers have been made by tabloids, and not much else,' she says. 'It will rule you and consume you and destroy you.' Her one concession to the onus of fame is that she's essentially unwilling to have children. If she meets a man? If she meets a man who shares her spiritual beliefs, and they marry? Then, maybe. 'I know it's old-fashioned,' she says.

Carey classifies her past relationships as dysfunctional (she refuses to talk about current relationships in public) and is extremely wary of gold-diggers. But children? 'You bring them into the world with the handicap of being a famous person's child. Fame has to be something you really want. I asked for this, I have to live with it - whatever that means,' she says. 'But a child you bring into the world doesn't say, "I want to be the child of a famous person," to then be attacked by the world. I feel bad for kids who have to go through that.'

Carey's thoughts on children stem in part, at least, from how acutely she still feels the pain of her own childhood. ‘As a child who grew up without a lot of money and who grew up without a lot of security - or any sense of it, really - I do operate from that,' she says. But she has money now, and hangers-on know it. 'They think they're dealing with Bill Gates, Jr,' she sighs. 'That's how a lot of people look at me, whether they're related to me or not, and it makes me feel bad. I have to work for this. They think I just walk around going, "La-la-la" and money falls out of the sky.'

She deals with her present fortune and past poverty with a combination of self-help work and, of course, music-making. Some publications used Carey's admission that she'd been in therapy to indicate that she was a bit, well, not quite all there. 'People have been going to therapy since whatever, since the beginning of time,' she says. 'I think being able to say I've gone to therapy is something an enlightened person would say, not someone in denial.'

There are other areas is in her life when she's had to deal with that narrow-mindedness - none more so than on issues of race. 'I'm in a room a lot of times with only white people, and they forget and say things that are offensive - and I have to put them in check,' says Carey. and I'm sitting there and I'm thinking I'm either going to have to leave or I'm going to have to talk about it.' But Carey won't be pulling a Nina Simone and renouncing the US any time soon. ‘After travelling the world, I think racism, or ignorance, is everywhere,' Carey explains. 'It's in the US, Italy, France... Even in certain Latin American countries, where it's a very mixed society. It's everywhere.'

While my evening with Carey is almost over, hers - at just gone l0pm on a Friday night - is only beginning. 'My guitar teacher's coming around,' she says. Carey's fingers have little blisters from practice. Then, later, at around 1am, she'll go out and get dinner with a friend at a place that stays open late because she's a good customer.

Mariah Carey has had the best-selling album in America of 2005. And even though she's spending lots of money to make money (she needs to buy property in Los Angeles because the hotel bills are ludicrous when she and her entourage head out west), she's now got enough cash in the bank to coast out her days.

But while she could buy a sunny little island somewhere and make an album whenever she feels like it, instead she has her new Elizabeth Arden perfume deal (she loves working on the perfume, and the perfumers say she has an excellent nose); a new deal with Gillette razors, in which we'll be exposed to many pictures of her fabulous legs; a jewellery and accessories line; and deals with Pepsi and Motorola. She is also working on a new film, called Tennessee. She has become the celebrity that puzzled and, rightly, troubled her when she was younger.

Carey is working all the time - extending, as the marketers say, her brand. Why? Because the little girl who was so afraid of staying poor, whose grandparents disowned her mother for marrying a black man, who has hours and hours of beauty school under her belt and whose mother didn't own a house until Carey bought her one, will never be penniless again. The princess in the tower might finally be happy - especially now, having just been triumphantly vindicated as an artist, but you wonder if she will ever find the security she still so craves. You hope so. Back downstairs at her thick, gilded front door, Carey is wide-awake and ready to get to work. ‘Air kisses!' she commands by way of goodbye, her face beaming.

Mariah Carey's new single, Say Somethin', is available now on Mercury Def Jam.