The announcement yesterday that Esa-Pekka Salonen, newly appointed principal conductor and artistic advisor of the Philharmonia Orchestra, will launch the opening week of concerts at the refurbished Royal Festival Hall in June with a performance of Mahler's third symphony both completes the circle of a great showbiz story and cements what the Philharmonia chairman has called the "longest courtship in music history". Back in 1983 Salonen was a late substitute for Michael Tilson Thomas, who had been due to conduct a Philharmonia performance of the Mahler. Salonen was then a 25-year-old best known as an uncompromising modernist composer. He had never conducted the work and had barely skimmed the score. But, after a few days of preparation, he went on an understudy and came off a star. One of the reasons the concert went so well, Salonen says, was that "it wasn't a make or break thing for me. I wanted to do my best, but I wasn't planning on a conducting career." The next day he realised things had changed. "The phone and fax went mad with all sorts of very important people from the music business. With hindsight, I am grateful that I didn't know who they were."

At home in Finland he became a tabloid celebrity, which he justified at the time by saying it was good for a classical musician to be known to the wider public, but really, he laughs today, "it was all vanity". Although Salonen is now nearing 50, the music marketeers still love to promote the boyish good looks of the leather-jacket-wearing father of three who likes to do interviews in pubs. And while he is surrounded by the same protective flummery as all maestros, he remains a reassuringly normal champion of some of the more dauntingly esoteric areas of contemporary classical music. He almost turned down his Philharmonia big break because he was called the morning after a late night with friends in London. "Yes, I did initially tell my agent to get lost. But thankfully, a couple of hours later, I changed my mind."

After the event, it took some time for him to change his concept of himself "from a composer who conducted to something different". He did continue to compose, but his conducting duties soon eclipsed his own work, as the Philharmonia appointed him principal guest conductor and he became music director of the Swedish Radio Symphony Orchestra. In 1984 he made his debut with the Los Angeles Philharmonic Orchestra, and in 1992 was made its music director. Expectations in Los Angeles, after a period of drift under André Previn, were huge. Billboards welcomed him to the city and one newspaper ran a headline calling him "The Great White Hope". It is generally agreed that he has pulled it off, elevating the LAPO to challenge the supremacy of the American "big five", especially in the post-19th-century repertoire. In 2006 he was named American "Musician of the Year".

It all seems a long way from his Helsinki childhood and early music education, but Salonen says his training remains invaluable. He was born in 1958 to parents who were music lovers but not musicians. "There was a certain amount of classical music in the house, but I didn't really grow up with it as such." His parents tried to send him to piano lessons, but he refused to go - "girls played the piano" - and instead found his own precocious and unexpected route into classical music after hearing Messiaen's Turangalîla-Symphonie on the radio. "I was about 11 and couldn't believe there was music like that. It was so shocking. I'd heard Beethoven and Brahms and Sibelius. But here was music behaving in a way that was completely alien. I think that is when I wanted to compose because, if that's what you could produce, I thought it must be the most fascinating thing on earth."

At music school Salonen studied French horn and composition before he was advised by a teacher to start conducting because composing was not a realistic way to support a family. "Saying that to a kid of 15 feels like the most irrelevant comment, but the fact is that composers mostly struggle. Which, of course, is crazy and tells us that the system is somehow not working." At the Sibelius Academy in Helsinki, Salonen formed the avant-garde music association Korvat auki! (Ears Open!) with fellow students Magnus Lindberg and Kaija Saariaho. He describes it as "highly idealistic and based on the notion that new music, particularly new serial music, made you a better person". They set up their own concerts in remote suburbs where they sometimes ended up playing "Kagel programmes to two old ladies who had come by mistake". And while they were the young avant garde of Finland, "looking back we were concerned with introducing principles to music that were already 20 years old. But I'm not embarrassed by any of it. Youthful arrogance can be excused."

While Salonen pays tribute to his teachers in Finland and, later, Milan, he reserves special thanks for the late Polish composer Witold Lutoslawski, who, although technically not his teacher, was an important mentor. Salonen has conducted premieres and award-wining recordings of his work. "We talked about lots of things, not just music. I still miss him a lot; and when I've done something that gives a - however fleeting - moment of satisfaction, I wish he was still around to share it." Salonen says that, as his conducting career blossomed, it released a side of him "that had always wanted to conduct Bruckner. But I still thought of myself as a hard-nosed, avant-garde person and this ultimately became something of a crisis."

By the late 1980s he had stopped composing altogether, having come to the dispiriting conclusion that "I didn't like my own music. So why not change it? It wasn't that easy. I had to go through a complete overhaul, and being in California was part of it." There was a moment one Californian morning in the early 1990s when he realised that he was not only happy but, "for the first time in my life, free to do whatever I wanted in terms of my own work. There didn't seem to be any obligation attached to it. That was the moment when the two sides of my music came together and the conductor could share the same taste and repertoire as the composer." Since then he has consciously taken time out to compose, including a year's sabbatical from conducting to write his epic LA Variations in 1996, and in the 2006-07 season he will have achieved a 50/50 split between composing and conducting.

He says the "deliberate ivory-tower state of mind" that classical music has suffered from since the war, when the avant garde deliberately isolated themselves from the general public, "might have killed music. As a kid I bought into it. It was very attractive to belong to a select group of intellectually agile people. No one really cared about what me and my friends were doing, but that didn't matter." But as you get older, he says, you realise that art is not about being right or wrong. "It works on many different levels, including the deepest emotional levels. If I'd even mentioned 'emotion' in my 20s, I'd have been kicked out of the gang. It took me 25 years to discover the simple fact that the best art moves you."

Salonen's new piano concerto is premiered in New York next week and he will conduct it at the proms in the summer. He wishes it might be seen as just another form of contemporary music. "A new album comes out from Radiohead and there is tremendous excitement, rumours on the internet and all the rest of it, about Thom Yorke's new songs. The same excitement should be there about what the new Tom Adès might sound like. So much new music is actually capable of creating this kind of excitement." He welcomes the increased downloading of classical music as both "great for documenting live concerts and a way in to new music for a tiny investment". His own works have been bestsellers on iTunes and next month he will demonstrate how he composes on a Mac at the Apple Store in Santa Monica. "I've looked at the playlists of people who've bought my work and you see things like Radiohead, a bit of world music and a Beethoven string quartet. It's breaking down compartments."

He found the Californian response to music refreshingly open-minded compared to Europe, where there has been "suspicion about things that are not in your compartment. When I arrived in LA, I learnt that things I thought were universal weren't. I had a list of great art that included Shakespeare and Beethoven and Thomas Mann and Rilke. But in a city where 160 languages are spoken, the European top ten is not the only option. I realised there were other lists, which took me back to square one in asking myself what it is that makes art interesting and important." He cites collaborations with director Peter Sellars, composer John Adams and video artist Bill Viola - "whatever you think of their art, nobody can deny they are independent spirits who have their own voice" - as opening up possibilities for him.

What will his LA experience contribute to his approach in London? "I don't have a master plan in my back pocket, but I am interested in interdisciplinary collaboration. Some of the most satisfying things I have done in LA have been collaboration of some sort." He mentions a Berlioz project with Simon McBurney and a Tristan project with Sellars and Viola. "It changed my life. To have someone like Bill Viola take a look at something we find so familiar, with eyes that are not jaded, was like seeing it again in true colour."

Salonen says that when he spoke "seriously" to the Philharmonia about taking up the post, the need to find "new ways to further the cause of classical music" and engage with the new media were on both of their agendas. "It's all very challenging, but not impossible. The greatest risk to classical music is not to respond to the culture until one day it just stops. There are some differences in the artistic structures and traditions of LA and London, but the approach that worked in LA was not to dumb down, but to do the opposite, to challenge the audience. And people responded. Nobody likes to be talked down to and we ended up with sold-out concerts and standing ovations for programmes of Liza Lim, Xenakis and Salonen. It didn't happen overnight. But the only way forward is to reinforce integrity rather than play The Nutcracker again and again. The only distinctions are between good and bad, and people know when they get the real deal."

Inspirations

Olivier Messiaen
Witold Lutoslawski
Peter Sellars
John Adams
Bill Viola