New York is a city of five boroughs – the Bronx, Brooklyn, Manhattan, Queens and Staten Island. It is a measure of Michael Bloomberg that during his years as mayor he surveyed this territory and imagined a sixth.

The “sixth borough” is the term Mr Bloomberg uses to describe the city’s waterfront. This shoreline runs along 520 miles of ocean, inlets, rivers and bays – an expanse five times the length of the Anglo-Scottish border – and by the time Mr Bloomberg took office at the start of 2002, it had seen better days.

The old waterfront – the one that moved the poet Walt Whitman to sing of a “city of ships” in the 19th century and sent 3m troops on their way to victory in the second world war – was no more. Port traffic had been shifting for decades to roomier places where it was easier to unload big shipping containers, and the manufacturers who once crowded close to the docks to transport their goods had largely gone in search of lower-wage pastures. As Mr Bloomberg took his oath of office, much of the area smelt of death – a result of the fires that raged at the World Trade Center site nearby.

Mr Bloomberg, however, saw something on the New York waterfront that was not there – yet. He saw graceful new parks and glittering residential towers rising from old factory sites, and a new graduate school sitting on an island in a river. He saw bike paths, kayaks and canoes. He saw a new New York built with the old port as a panoramic backdrop, which would offer the culture, comfort and convenience needed to attract the talented people from around the world who would maintain the city’s supremacy as a financial centre and develop the new businesses of the high-tech future – terrorists be damned.

Mr Bloomberg began building that city as mayor – and its outlines can be observed today. Fifty-storey apartment buildings offering dramatic views of the Manhattan skyline are sprouting from the “outer boroughs” of Brooklyn and Queens. Ferries plying the East River have carried millions of passengers since a new service started two years ago. There is even a park on the Brooklyn side of that borough’s eponymous bridge where a wine bar can be found amid marsh marigolds, swamp roses and pussy willows.

Yet such was the scale of Mr Bloomberg’s vision that after nearly three terms in office – the last won only after he prodded the City Council to scrap term limits – he isn’t done. During his tenure, the zoning rules for 37 per cent of the city were changed to permit redevelopment by the private sector, and work on some of the biggest projects is just getting started as he prepares to leave office at the end of this month.

Many of these developments – typically of the “mixed-use” kind – are years away from completion. At Hudson Yards, a city within the city – complete with its own subway extension, parks, plazas, futuristic skyscrapers and room for tens of thousands of new residents – is being built around an old railcar storage area next to the Hudson River on Manhattan’s west side. In Queens, all 62 acres of Willets Point – much of it now given over to ramshackle car repair shops – will be razed so sewers and housing can be brought to an area F Scott Fitzgerald compared to a “valley of ashes” in The Great Gatsby.

As a result, New York is about to undergo a curious changing of the guard. Mr Bloomberg, 71, will give way to Bill de Blasio, 52, an unapologetic leftwinger who swept to an easy victory in this year’s mayoral election by attacking income inequality in the city and the aggressive tactics of Mr Bloomberg’s police department. But even though the Bloomberg administration will officially end at that point, the Bloomberg era in New York City will continue for decades to come – in all six of the “boroughs”.

“He is going to define the city for the next 25 years,” said Mitchell L Moss, a professor of urban policy and planning at New York University, and a campaign adviser to Mr Bloomberg in 2001. “It doesn’t matter who the next mayors are. They are still going to be attending groundbreakings for projects he started,” he said.

If all goes according to plan, the development to come will also produce a city with a few more Michael Bloombergs in its midst. The mayor believes talent attracts capital more easily than the other way around, and his six-borough experiment in urban living is designed to appeal to people like himself – hands-on entrepreneurial sorts who will be able to shake up the city’s economy in their time just as he did in his.

…

Mr Bloomberg, who grew up in a suburb of Boston, came to New York in the 1960s and settled into a city that profited from its position at the crossroads of capitalism. At the port, New Yorkers handled goods. On Wall Street, they directed capital flows. Either way, they were in the middle of things – and took their cut.

Mr Bloomberg did well in this world. He made his first millions as an equities trader at the Salomon Brothers investment bank, now part of Citigroup. But he made his billions after he was fired by his Wall Street employer and started a company of his own that sold information via computer terminals bearing his name.

The lesson for Mr Bloomberg was that there was more to life than Wall Street-style wheeling and dealing. In his 1997 business memoir, Bloomberg on Bloomberg, he wrote that “the Street promised vast riches” but that he had “read of few great fortunes having been made there”. He concluded: “Great financial success comes from starting businesses with concrete products in the real world, building jobs, creating value and helping people.”

As mayor, Mr Bloomberg has tried to steer New York’s economy in this more concrete direction. He remains a friend of the local financial industry but he has been a relentless advocate for technology. He has pushed New York to diversify, putting special emphasis on expanding the pool of people in town with the kind of training in engineering and applied sciences he believes will lead to successful start-ups.

The problem facing New York is that the Bloomberg of the future – the man or woman in the city with the next big idea – could easily wind up going somewhere else. Creative people need only plug in their computers to get started these days and that gives them less incentive to pay their way in a big, expensive town such as New York.

Robert Steel, Mr Bloomberg’s deputy mayor for economic development, said New York is being challenged by smaller cities known for their quality of life – such as Boulder, Colorado; Austin, Texas; and Seattle – in much the same way as it fought for talent a couple decades ago with places such as Los Angeles, Chicago or Boston.

“Given the connected nature of things …New York now competes with all those places,” said Mr Steel, who came to Mr Bloomberg’s City Hall after stints at Goldman Sachs, the Treasury Department and the old Wachovia bank, the latter as chief executive.

The mayor’s response can be seen in so-called quality of life improvements all around town: the bicycle-share programme he introduced this year, the pedestrian plazas of Times Square and even in his health-oriented fatwas banning smoking in bars and restaurants, and requiring food-service providers to show calorie counts on their menus.

But Mr Bloomberg’s war for young talent is being fought most relentlessly on the waterfront. It is getting greener and groovier by the minute as Mr Bloomberg hands over swaths of his sixth borough to joggers, bikers, boaters and plugged-in hipsters who do not care to drive (turning the work of the New York master-builder Robert Moses on its head in the process). In these environs, Mr Bloomberg’s New York is coming to feel more like a campus than a city – a sensation that will only grow in the years ahead.

A new graduate school of applied science – Cornell NYC Tech – is being constructed on Roosevelt Island, which sits in the East River between Manhattan and Queens. Mr Bloomberg held a contest to get it started – offering $100m in city money to help the builders of what the New York Daily News has called his “genius school.”

Cornell University and Technion-Israel Institute of Technology won the bidding in 2011, and hope to open the Roosevelt Island campus in 2017 and complete it by 2037. For now, the school, like so much Mr Bloomberg has started, remains below the radar, offering graduate degrees in subjects such as “connective media” – technological solutions for media types – to a few dozen students in Manhattan’s Chelsea section. But the city projects it will one day accommodate 2,000 students, making New York far richer in engineering talent.

…

The irony is that this is not doing all that much for the mayor’s personal popularity. In November, New Yorkers gave Mr Bloomberg – a registered independent – the political equivalent of a Bronx cheer, handing 73 per cent of their votes to the Democratic Mr de Blasio. This was the highest percentage for any mayoral candidate in more than a quarter century.

Mr de Blasio played on the sense of estrangement that many New Yorkers obviously feel from their billionaire mayor. He scored points with the public by highlighting the persistence of poverty – the number of people living in homeless shelters has risen to record levels – and criticising police for “stop and frisk” policing strategies that angered minority communities.

“There has not been a mayor in modern New York history who has spent less time with rank-and-file New Yorkers,” said Joel Berg, executive director of the New York City Coalition against Hunger. “I don’t think there was a five-borough, every neighbourhood focus. You need a feel for these places. You need to spend time.”

It is easy to see how Mr Bloomberg might have seemed fey to some New Yorkers as he focused on bigger-picture issues, such as the fate of an imaginary borough with no actual voters, technically speaking.

But city officials say he has been unfairly blamed for economic problems – such as the pressure on wages for unskilled work – that result from global or national forces. They point out that the city has been adding population and jobs in recent years, and maintain that the mayor’s policies will lead to more gains.

As for the sixth borough, there are no strong indications at this point that Mr Bloomberg’s work will face any substantial challenge from the new mayor. Mr Bloomberg has probably reached the point where any remaining threats to his legacy are more heavenly in nature.

Mr Bloomberg has pushed his city closer to the water – and that raises inevitable questions during a time when some fear global warming is to blame for rising seas and the kind of flooding New York endured last year during Hurricane Sandy. City officials express confidence that refinements in the building code and improvements in the port will help New York cope better in the future.

But only time and the arrival of another big storm will tell, which is a reminder that for all his achievements, and his considerable resources, Mr Bloomberg will still have to pass muster in the end with powers higher than those found in any borough, actual or imagined.