When Jamie Dimon strides up the stairs into the Grill Room of the Four Seasons at noon, the New York restaurant is only half full. But, being the Four Seasons, perhaps the most famous power-lunch venue in the world, it is half full of some of the biggest names in American business.

As he shakes my hand, Dimon glances over to the corner table that used to be reserved for the late Philip Johnson, the architect who designed the restaurant's elegant interior. There sits Pete Peterson, former US commerce secretary and co-founder of private equity giant Blackstone, in conversation with Martin Sullivan, the British-born chief executive of American International Group, the world's biggest insurer. At other tables are Edgar Bronfman, now head of Warner Music, and Mort Zuckerman, the real-estate magnate who owns New York's Daily News.

Even in this company, Dimon stands out. The tall, boyish-looking 50-year-old was recently elevated to chief executive of JPMorgan Chase, one of the most powerful financial institutions in the world.

He is a darling of the media, who love his fast-talking, unzipped style and the story of the kid from Queens who rose to the top of the bank founded by the Morgans and the Rockefellers. They also love the tale of the feud between Dimon and his former mentor Sandy Weill, who built Citigroup into the world's biggest financial services company.

Eight years ago, Weill sacked Dimon. Now he is running Citigroup's archrival.

As Dimon studies the menu, he recalls the many meals the two shared here, including a lunch in December 1999, a year after he was sacked. Dimon had called Weill to make peace and the resulting lunch was an FT scoop. "There it was on the front page: Heavy Russian casualties in Chechnya; Record US trade deficit and... Dimon has lunch with Weill. It was very funny."

After the waiter has gone through the specials, Dimon says he would order steak tartare if it was dinner. "I will do the steak tartare right now for you Mr Dimon if you would like," says the waiter, in a tone suggesting he would do anything, including the chef's head on a platter, if Mr Dimon would like. Dimon hurriedly turns down the offer and asks for tuna sashimi, Dover sole and a diet coke. I go for the lobster risotto and crabcakes.

Dimon was already a star when he was cast out by Weill and was bombarded with job offers, including internet companies, Home Depot and, it is said, Barclays of the UK. But Dimon took his time. For the next 18 months, he travelled, read - particularly history - learned to box and pondered a complete change of career. "I thought about teaching, but I wasn't ready yet." One of the most senior active Democrats on Wall Street, Dimon also considered politics. "But it's just not really me... it's too partisan."

In the end he stuck with what he knew and accepted the chief executive job at Bank One, the Chicago-based bank that four years later he sold to JPMorgan for $58bn.

Bank One was no small business. But he is in a different league now. "Most people who run large companies feel a tremendous responsibility. There's a joy to that and there's a burden to that. Like having children." At JPMorgan he feels an obligation not only to its 170,000 employees and the shareholders but also to his predecessors and the bank's history. "If you look at the depth and the breadth of the company and its relationships around the world that go back well over a hundred years, it's staggering."

As the food arrives, Dimon admits to being a news junkie, waking at 5 or 5.30am and spending up to an hour and a half reading three newspapers: the FT, The New York Times and The Wall Street Journal. He is very interested in public policy and is happy to share his views on most issues - apart from the Iraq war. He bemoans the lack of political leadership that has left the US with "almost no energy policy" and says it should be "relatively easy to fix" the US pensions crisis. "I think the much more complex one is medical [benefits]. It's bigger and it's growing faster."

On foreign policy, he believes the US could play "a very positive role in global development" and should take a lead in ensuring that globalisation and free trade work. "Globalisation done well is great for the world. But there are lots of winners and losers."

Dimon eats as fast as he talks and has already polished off the tuna while I am only half-way through my risotto. Returning to his 18 months off, he says he has only one regret. "That I didn't take up a musical instrument." He is now having guitar lessons every Sunday, though he can't always make them and hasn't always done his practice. "I told my instructor the other day 'I'm 50 years old, I'm CEO of a big company and when I see you I still feel guilty I didn't do my homework.'"

Dimon's image is of a street-smart outsider who prefers computers to clients and has no time for Wall Street pretensions. The reality is somewhat different. Born into a comfortable middle-class New York family - his father was a broker with what became Citigroup, where he still works - he sailed through Tufts University and Harvard Business School where he met his wife Judy, a former consultant with Booz Allen Hamilton, who now sits on a number of not-for-profit boards. (Some say she is even smarter than he is.)

Dimon is a well-known non-golfer - a rarity on Wall Street - but not, he insists, for social reasons. "I just never took it up. I didn't have time living in the city with a young family." His main sport is tennis, which he plays with his three daughters. He does his fair share of black-tie dinners and is active in philanthropy, though mainly poverty, health and public services rather than the arts.

As I start a long battle with my monstrous crab cakes, he recalls how some Chicago firefighters after September 11 told him about new cameras that could see through smoke. "The mayor couldn't afford them so I called him up and said my wife and I would buy one for each of the hundred firehouses in Chicago."

Although he is worth several hundred millions dollars - mainly in JPMorgan stock - Dimon has none of the trappings: no yacht, no fancy cars, not even a house in the country. Because his third daughter is still at school in Chicago, the family home remains there and he flies back from New York every weekend. "It's a pain," he admits, even with the private jet. But his elder daughters told him it just wouldn't be fair to move the youngest to New York.

Dimon isn't a computer geek, but he does appear to be as focused on the nuts and bolts of business as on strategy. "Perfect execution of a second-rate strategy is better than second-rate execution of a perfect strategy. It's very easy to sit in corporate headquarters and pontificate about strategy when someone is eating your lunch."

While he resents his reputation as an obsessive cost-cutter, in conversation nothing seems to excite him more. "We all know that corporations can waste a tremendous amount of money. It's destructive. It's wrong." His campaign against waste has included axing company cellphones, a move that infuriated senior executives who must now spend hours claiming work calls on expenses. Is this not a bit trivial?

"People say: 'It's too small - don't pay attention to that.' I say: 'That's the road to hell.'"

I ask whether the highly paid executives in JPMorgan's investment bank should be worried about further cuts. No, he says, but then rather suggests otherwise: "Our entire company is tremendously bureaucratic and we are perhaps half-way through fixing that." But he adds that the group has also invested heavily in the investment bank and increased pay.

He strongly rebuts accusations that he is better at squeezing businesses than growing them but pleads guilty to charges of micromanaging. "That may be somewhat true."

Given Dimon's history at Citigroup and Bank One, there is constant speculation about his next big deal despite his insistence that there is still lots of work to do integrating Bank One. JPMorgan is frequently named as a potential purchaser of Standard Chartered, which has an attractive business in Asia. Although he says "we could be a very successful company and never be in China," he admits the big gap in the group's portfolio is consumer banking outside the US, particularly in Asia. "I think throwing money at it would be a terrible mistake. We're going to have a thoughtful strategy and we may tell the world about it when we've figured it out."

But JPMorgan will play its role in continued industry consolidation. "We will be in the acquisitions business. Hopefully intelligently, and that will include international."

There is one particular deal that some astute Wall Street observers believe will happen - a merger of JPMorgan and Morgan Stanley to recreate the House of Morgan, forced apart by legislation in 1933. But this, at least, Dimon appears to rule out. Without naming names, he says it would "not be feasible" to combine two big investment banks because of the loss of staff and clients.

By this stage, Dimon is badly behind schedule. He says he needs to get back to his job ensuring JPMorgan is a "vibrant, healthy company, which is the best thing I can do for the world".

Part of that job is schmoozing the biggest names in American business. And, luckily, many are still in the room. He heads over to Pete Peterson and Martin Sullivan for a chat, then has a last quick look round the other tables and strides off down the stairs.

David Wighton is the FT's New York bureau chief.

The Four Seasons, New York

1 x small crudites

1 x blue fin tuna

1 x lobster risotto

1 x Dover sole

1 x crabcakes

1 x diet coke

1 x mineral water

2 x espresso

Total: $195.08