'I refuse to get too bogged down in the business of being a singer," says Karen Cargill, internationally renowned Scottish mezzo soprano.

We're talking about how she manages to keep her cool on the world's biggest stages. "Let's face it" - she leans over the table, whispering like a naughty secret. "Basically, I'm just a musician who stands up there and delivers words and melody. I don't save the world, I don't save lives. I try to give people something for however long I'm on stage. Some listeners might hate it, some might love it, but they'll all have had an experience and that's art. That's what we do."

Over the past few years Cargill has been increasingly recognised as one of the great mezzos of today. Her voice is irresistible: velvet, poetic, generous, full of compassion. She has been championed by a top tier of conductors including Simon Rattle, Donald Runnicles, James Levine, Bernard Haitink and Yannick Nezet-Seguin; she sings lead roles at New York's Metropolitan Opera and London's Covent Garden. Last year her sensational recording of Berlioz with Robin Ticciati and the Scottish Chamber Orchestra drew extensive critical acclaim.

And still Cargill is as frank and personable as ever. If American mezzo Joyce DiDonato is "just Joyce from Kansas", Cargill is "just Karen from Arbroath", with a hearty laugh, an unguarded warmth and a great knack for storytelling. Our interview is meant to be a quick coffee but clocks in at nearly three hours. Cargill is heading out for a girls' night later and has just had her hair done; she looks glamorous and utterly relaxed. "With age I've learned to be kinder to myself," she says. "This is my voice and I can only do what I can with it."

She is in Scotland this week and next to open the Scottish Chamber Orchestra season singing Mahler. First is the Fourth Symphony, whose finale features the song Das himmlische Leben (normally sung by a soprano); next week we hear Kindertotenlieder, some of the most gripping, devastating music Mahler ever wrote. Later in the season Cargill is back with the SCO to sing Das Lied von der Erde, and she recently released an album of songs by Mahler and his wife Alma - a tender collection to which Cargill and the pianist Simon Lepper bring earthy, instinctive warmth. "I was barefoot for most of the recording," she remembers. "I sat on the floor while we listened to the playbacks... I wanted to be as grounded and as relaxed as possible. I hope that comes through." (It does.)

Why so much Mahler? "Well, it's where my voice is at," Cargill replies. "There's a real resonance for the mezzo sound; he wrote so well for lower-set voices. In Kindertotenlieder, for example, the gravitas that you need for texts about the death of children... Oouf. Mahler was such a complex person and you have to be able to bring out that depth, that darkness. You have to capture the broodiness. Mezzos are always broody!"

She has only good things to say about Robin Ticciati - his "wonderful, wide-eyed freshness" and "enthusiasm to explore different soundworlds" - and describes working with the SCO as being "surrounded by friends". It's unusual for such a small orchestra to play Mahler but here, she says, "there is no fear, only excitement".

As for singing to a home crowd? "That has its own pressures. Away from home I only have to think about myself. I can be selfish and focus entirely on my routine, my voice. At home I have to be mummy and wife."

But evidently Cargill is resilient. She sang until she was 37 weeks' pregnant, took five weeks off after the birth then did a televised Prom singing Das Lied von der Erde with Donald Runnicles. When she later watched the recording she could "see the hormones" in her eyes, she says. "I was singing 'ewig, ewig'" - the repeated, emotion-drenched "forevers'"that close the work - "and the tears were streaming down my cheeks. I had no idea I was even crying."

Her son Adam is now six, and balancing engagements at the Met with school pick-ups in Bearsden has been a tad tricky. She didn't work during his first six weeks at school - "just to make sure I could be there to see the excitement and the fear. I told my agent that I wanted that time off back when I was pregnant: I knew I would want to be with him during that huge transition. He's the best thing I've ever done. If I'd missed it I would never have forgiven myself."

In truth, making the time to be a mum doesn't seem to have hindered Cargill's career much. Now she simply cherry-picks the best invitations: Ariadne at Covent Garden; Berlioz in Cleveland; Die Meistersinger at the Met; song recitals at the Carnegie and Wigmore halls. "When I was at the Met last season doing the Ring Cycle then Trojans... That was a long time to be away from home. I love opera, but I also love spending four days of intense rehearsal, picking away at a score and then - boom - doing the concerts." Ideally she would sing one or two operas in a season and plenty of concerts. "That would let me have an almost-normal life. More time at home in Bearsden, more time dancing around my kitchen with Adam, washing my clothes in my own washing machine... It sounds daft but these things mean a lot."

When she's on stage, though, there is nowhere else in the world. Watching Cargill perform is transfixing because she is so extraordinarily candid. Nothing is held back, nothing is over-baked or contrived. Her expression is sincere, heartfelt and natural-sounding. She says that in every concert she has at least one moment when she looks out to the audience and thinks, this could be the last time that I ever do this. "I let the spirit take me. Oh wow," she cringes, "that's a very 'drama teacher' thing to say. But really, I do. It might be a harmony or a pause, or a gesture that the conductor does or just a moment of silence. But something incredible happens in these moments - they mean the world. You'll probably be able to tell now because I go all meditative and break into a sort of cheesy grin..."

That ability to reach total abandon didn't come instantly. She used to suffer panic attacks, even be sick before performances. "That was in the early days when I thought you had to go out there and prove yourself every time. I had a hugely important conversation with [Scottish soprano] Lisa Milne, who underlined that this singing thing can't be about proving yourself. It's simply about telling the story that's on the page as best you can. It's very Zen, actually... I wish I was that Zen all the time!"

Does it matter what critics say? "Oh God, yeah! I don't think any singer could truthfully tell you otherwise. The key is to not let it affect you either way." I mention that she seems wonderfully unpedantic about her voice. "My husband will tell you otherwise! In the beginning I was a bit obsessed. I'd be up at stupid o'clock singing top Cs. Nick would come down and say, 'it's 6.30am; you haven't slept and the neighbours are going to put a brick through the window. Why are you torturing yourself?' He's the one that keeps the sanity in the moments of insanity."

But Cargill is clearly grounded herself. Her dad is a plumber in Arbroath and her mother worked in a bank. "Nobody in my family is particularly musical, but they've been everywhere to hear me, including New York a few times." She calls them her quality control spies. "I never planned to be a professional singer," she says. "I didn't imagine singing opera, let alone Wagner."

Even when she was studying at the Royal Conservatoire of Scotland she thought she would probably be a school music teacher. When did she realise that she wouldn't be? "I'm still not sure if I've realised that! I never had a set goal of where I wanted to be. I just enjoyed the journey, thinking 'this is pretty damn cool...'"