"We need leadership from the White House - and over the past four years, President Barack Obama has taken major steps to reduce our carbon consumption, including setting higher fuel-efficiency standards for cars and trucks," Bloomberg said in an opinion piece on his website.
After the East Coast devastation wrought by Superstorm Sandy, Bloomberg said the possibility that Sandy resulted from climate change had made the stakes of the election that much clearer.
Both Mr. Obama and Republican presidential candidate Mitt Romney had sought endorsement from the politically independent and nationally recognizable mayor. It was a major boost for the president, who is spending the campaign's final days trying to win over independent voters.
The billionaire businessman and former Republican said Romney "would bring valuable business experience to the Oval Office," but said Romney had reversed course on issues like health care and abortion.
"If the 1994 or 2003 version of Mitt Romney were running for president, I may well have voted for him," he said.
Bloomberg did not endorse a presidential candidate in 2008.
Romney advisers, asked about the endorsement, dismissed it as inconsequential and suggested it would have no bearing on the race outside of New York City.
Presidents Bill Clinton and Ronald Reagan both found success while their parties were out of power in Congress -- and President Obama can, too," Bloomberg wrote. "If he listens to people on both sides of the aisle, and builds the trust of moderates, he can fulfill the hope he inspired four years ago and lead our country toward a better future for my children and yours."
The Associated Press contributed to this report.