In 1981, squeezed out of the bank in a merger, he started Bloomberg LP, the financial news and information company used by Wall Street's trading firms. It has made him rich.
Since then, he has donated more than $2.4 billion to a variety of causes and organizations. About half of that -- more than $1.1 billion -- has gone to Hopkins, which named its school of public health after him.
"He's driven by the metric of how do we make the most impact in terms of improving health and saving lives," said Dr. Michael J. Klag, dean of the Bloomberg School of Public Health. "Mike is a data-driven guy, he's a numbers guy."
Klag credits the businessman's investments in public health with paying big dividends.
"New York is now the healthiest city in America because of what he's done," Klag said.
It's also one of the safest, with the lowest murder rate of any big city in America. And traffic deaths and fire deaths are the lowest since the city started keeping records in 1916.
"Parents from around the country used to dissuade their kids from moving to New York because it was dangerous," Bloomberg said last week. "That doesn't happen any more. Parents want their kids to move here because they're probably safer than where they're coming from."
Putting a dent in obesity
Bloomberg's focus on food is recent and the data to support it are not as strong.
"With tobacco, we have a pretty established set of policies we know are effective, but with obesity, it's still new," New York City Health Commissioner Dr. Thomas A. Farley told an audience at Fordham University in New York during a conference last fall on Bloomberg's public health legacy.
But talking about legacy is "a little presumptuous," according to Peter Zimroth, a lawyer with Arnold & Porter, which has represented the restaurant industry.
"The legacy is yet to be written on some of these initiatives," he told the conference, adding that the evidence that such efforts would reduce obesity was itself thin and could wind up being counterproductive. "There's a limited amount of capital, I think, that government has to engage in coercive measures," he said.
Bloomberg sees restrictions on food as a critical component to his public health mission, according to Klag. "He sees this avalanche of diabetes coming," he said. "That's why he attacks the sugar intake."
In reaching for opportunities, Bloomberg has lost his footing several times. In addition to losing his effort to ban large, sugary drinks, he failed in his bids to get a soda tax adopted and to ban the use of food stamps to buy soda.
The so-called soda ban provoked ridicule from some observers. "I think this is what makes liberals look like elitist bullies who think they know everything and can tell people what to do," Bill Maher said on his HBO show "Real Time." "You shouldn't have to clear what you eat with the municipal government."
A executive with the National Restaurant Association -- which represents 500,000 restaurant businesses across the country -- called the city's efforts at change "heavy-handed" and was effervescent over Bloomberg's most recent loss.
"We very much questioned the efficacy of putting a dent in obesity by restricting the cup size in restaurants in New York City that sold sugar-sweetened beverages," said Scott DeFife, the association's executive vice president for policy and governmental affairs. "We don't think that micromanaging food service packaging is the way to end obesity in New York City."
Bloomberg's attempt to limit the size of sodas inspired Mississippi -- whose 34.9% obesity rate is the nation's highest -- to pass an "anti-Bloomberg" law.
"It simply is not the role of the government to micro-regulate citizens' dietary decisions," Gov. Phil Bryant wrote.
But Bloomberg lashed back.
"'Saturday Night Live' couldn't write this stuff," he said about Mississippi's move. "We have a worldwide, nationwide problem on obesity. This year, more people will die from overeating than from starvation -- first time in the history of the world."
"How can somebody try to pass a law that deliberately says we can't improve the lives of our citizens?" he asked. "It's farce."
While his critics might accuse him of simply trying to expand his political power through his health initiatives, Bloomberg's colleagues say that's not the case.
Bloomberg's focus is simple and singular, explained his former colleague Dr. Tom Frieden, director of the Centers for Disease Control and Prevention.