Even by his own dynamic standards last week was a busy one for Cameron Mackintosh. One night saw the opening of his new production of Miss Saigon that had already clocked up more than £10m in advance-sales. The next day Mackintosh announced that he was buying two new London theatres – the Victoria Palace and the Ambassadors – to add to his existing seven-strong portfolio. When you consider that he has produced the three longest-running musicals of all time – Les Miserables, The Phantom of the Opera and Cats – and that he is worth an estimated $1.1bn (£658m), you might be forgiven for thinking of him as a mega-tycoon. Yet, talking to him this week in his Bloomsbury offices, I was struck by how he remains the same bubbly enthusiast I have known on and off for 40 years. You go expecting to meet Citizen Kane: what you see is Peter Pan.

Mackintosh, although now 67, has the unlined features and bright-eyed fervour of someone who has spent all his life doing precisely what he wants: putting on shows. He also brings to the acquisition of theatres the same enthusiasm, and microscopic attention to detail, he does to producing musicals. I've never forgotten how, after he bought the Gielgud theatre in 2006, he insisted I inspect the radically transformed ladies' loo: not a subject, I confess, on which I had any expertise.

Now he is excited by his purchase of the Victoria Palace and the Ambassadors (to be rechristened the Sondheim). "I love old buildings," he tells me, "and, if I hadn't been a theatre producer, I'd have been an interfering architect. The Victoria Palace is the best house designed by the great Frank Matcham from the point of view of intimacy and sightlines, and will have an extra stage depth of 20ft." Mackintosh is equally fired up by the new Sondheim and juggles a sheaf of architectural plans with the enthusiasm of a madcap Christopher Wren.

But what will the Sondheim be used for? "I want it," says Mackintosh, "to give a longer life to some of the best work from the subsidised houses in London and the regions. I've talked to the two Nicks [Hytner and Starr from the National Theatre] and they say it would have been perfect for the transfer of the musical, London Road. A show like the Young Vic's The Scottsboro Boys would also have been ideal for this space. Or it could have taken National Theatre Scotland's Black Watch. The idea is to run each show for eight to 16 weeks but not to plan too far ahead so we can take in shows from, say, the Royal Court or the Menier or the Chichester Minerva that deserve wider exposure."

Mackintosh quickly scotches the idea that the Sondheim will be used to try out new musicals. But, before we get on to the controversial subject of whether the musical is a declining form living off its golden past, I am intrigued to know how Mackintosh remains so sane and rooted in spite of his vast wealth. How can one not be intimidated by the knowledge one is a billionaire – especially when the information has been proclaimed on the Sunday Times super-rich list?

"For a start," he says, "I'm only worth a billion if I want to sell my shows or theatres, which I don't propose to do. But I've known sufficiently hard times not to be affected by wealth. I'm a war baby, I was brought up with rationing and my parents always had to struggle. I remember when I was sent to boarding school – Prior Park College in Bath – my father was asked how he was going to pay the fees and he replied: 'In arrears.'

"As a young producer, I also learned the hard way. One of my first shows, Anything Goes in 1969, lost £40,000 in unsecured loans and another one, Home With the Dales, lost another £10,000 because no one wanted to see it. And I've never forgotten the dagger in my heart of a show called After Shave, which I hoped would be a female Beyond the Fringe, and which turned out a complete disaster.

Miss Saigon is one of Mackintosh's biggest successes. Photograph: Tristram Kenton

"I survived because I never took on big responsibilities in my private life. In the early days I lived on two or three pounds a week and learned to cook – and I'm a good cook – because I had to. Even when I went on holiday, I stayed in other people's houses. Now I can take friends on holiday and, if I wanted to, could afford a private jet. But, if I'm taking a short flight to Scotland or the continent, I travel economy. I go on EasyJet all the time and I've got my travel pass for the underground, which I use whenever I'm dashing between different rehearsal rooms. Like Ed Miliband, I probably couldn't tell you my week's grocery bill but I use public transport for two reasons. It's often the most practical way to get around London and it's important for me to know what it means to someone to fork out £50 or £60 for one of my shows. I realise that the people who've made my fortune are people who have to budget carefully and who want value for money."

Mackintosh is, in no sense, a tightwad. He has set up a foundation that supports numerous charities, individuals and theatrical projects including an annual Oxford professorship at St Catherine's College. He even, in 1998, gave a handsome donation to the Labour party. When I ask if he is likely to repeat that, he offers an emphatic "No", adding: "I'll never vote for them again because of their incompetence." But, Mackintosh remains a benign capitalist and one who has never insulated himself from daily reality. "I'm privileged," he says, "to have had some success but I've never forgotten what it was like to queue for a half-crown gallery seat for Oliver! which is why I ensure that there are £20 day tickets for Miss Saigon and that the balconies in my theatres are as comfortable as I can possibly make them."

To use an old-fashioned term, Cameron Mackintosh is a good egg. He only bridles when I suggest that the musical, despite its economic buoyancy, is living off its back catalogue. For all the success of Les Mis, Phantom and Miss Saigon, new musicals, I suggest, are thin on the ground and recent much-touted entries such as Andrew Lloyd Webber's Stephen Ward and Tim Rice's From Here to Eternity have not survived long. There even seems little room for a modest show such as Betty Blue Eyes by George Stiles and Anthony Drewe, which Mackintosh originally produced and which had only a brief West End run in 2011.

"I disagree with you," he says. "To take the specific case of Betty Blue Eyes, the only problem was one of timing. We were too close to the royal wedding and the economic crisis. But the show's just been wonderfully revived in Colchester, is going on a six-month tour and has been a great success in America.

"What you forget is that a lot of the shows we now regard as classics didn't make it the first time round. Cabaret had only an eight-month run in London in the late 1960s, even with Judi Dench, and Sondheim's Sweeney Todd was not a big hit when it was first done at Drury Lane."

But don't musicals now have to be epic and expensive in order to succeed? Is there still room for shows such as Trelawny or The Card, both of which Mackintosh himself produced in the early 1970s? "Of course, there is," he says. "Two of the biggest hits in London right now are Matilda and The Book of Mormon. Both are medium-sized musicals, both slightly oddball and both attracting huge audiences because of the quality of the writing and imaginative staging. There are also far more venues for new musicals than when I started out. Today you can take a project to the Menier or Southwark Playhouse in London or to Leicester, Leeds, Sheffield or Chichester. In the old days, it was shit-or-bust but today there are far more opportunities. No producer, however, can put together a hit musical if the material isn't there."

Several times Mackintosh makes the point that producers can't instigate shows: they can only hope to find good material and make it better. But is it true that he interferes in every aspect of a production? "I suppose," he says, "I'm a bit of an 'eminence grisly' but my sole aim is to get the show right.

"I'm proud of the fact that I've taken a lot of big directors, such as Trevor Nunn and Nick Hytner, who were musical virgins, and introduced them to the form. I also supervise every single word of the script and every note of the orchestration and I'm at the knee of the lighting designer all the time. But, when the show's in rehearsal, I only pop in two or three times a week early on, then I let the director and choreographer get on with it. To be a good producer, you need to be able to distance yourself. If a director wants my approbation for everything they do, I know it's going to be a disaster."

What is fascinating about Mackintosh is that he's now in the third stage of a long career. First there was the perpetually hard-up young tyro who never really hit the jackpot until the revue Side by Side by Sondheim in 1976: a show he booked sight unseen since he missed a Sunday-night try-out in Wavendon by driving up the M4 instead of the M1. Then came the prolific middle period in the 1980s and 90s when shows such as Cats, Les Mis, Phantom and Miss Saigon helped transform the musical from a national product into an international franchise.

The cast of Les Miserables. Photograph: Michael Le Poer Trench

But he is equally excited by what he is doing now – in what he calls his "daddy period" – which is to offer new productions of old hits, or even the odd failure such as Moby Dick.

"I find it curious," he says, "that no one thinks twice about there being a dozen different Hamlets or King Lears but people seem sceptical about revived musicals. But I feel that, however brilliant the original production was, you can't expect a new audience to get the same frisson from it. That's why I'm doing Miss Saigon with a new director, Laurence Connor, and a totally fresh design team. I've also just opened a new version of Les Mis on Broadway. And I've got Miss Saigon opening in Tokyo in eight weeks' time, then it's off to Australia for Les Mis, then back here for the national tour of Brian Conley in Barnum and, next, Mary Poppins in Vienna in October.

"I actually thought around the time of The Witches of Eastwick in 2000 that I'd come to the natural end of my producing life. What I never foresaw was that the circle would complete itself and I'd be doing new versions of the shows of 20 or 30 years ago."

Like the rest of us, though on a bigger scale, Cameron Mackintosh is full of contradictions. He itemises his whirlwind activity over the next year with obvious relish. In the next breath he tells me that what he really wants is to be at home in the country: in his case the medieval Somerset priory he shares with his long-term partner, Michael Le Poer Trench, which they run as a thriving farm ("We've got two dairies and 600 or 700 cows"). He is a producer with every fibre of his being – even at school he was nicknamed Darryl F Mackintosh in a nod to a legendary Hollywood mogul – and yet offers a modest definition of his role: "My greatest talent," he says, "is to be a chameleon and, providing someone has written something interesting, to be the catalyst to make it work."

• This article was amended on Friday 30 May to correct a ticket price