"I'm a New Yorker and I know a con when I see one," he told the crowd about Trump.
Bloomberg, 74, an independent and former Republican stumping for Clinton, acknowledged differences with the nominee he was endorsing but said that it was important to put them aside.
"There are times when I disagree with Hillary. But whatever our disagreements may be, I’ve come here to say: We must put them aside for the good of our country. And we must unite around the candidate who can defeat a dangerous demagogue," Bloomberg said.
He also called Trump a "bomb thrower."
"Today, as an Independent, an entrepreneur, and a former mayor, I believe we need a president who is a problem-solver, not a bomb-thrower; someone who can bring members of Congress together, to get big things done. And I know Hillary Clinton can do that, because I saw it firsthand!" he said.
Bloomberg’s take on Trump comes on the heels of having weighed a run for president as a third party candidate, something he also toyed with in 2008. Ultimately, after his his chances of winning appeared to be slim, he decided against it.
Bloomberg has been both a Democrat and a Republican at different times in his career. He registered as a Republican before his first campaign for mayor in 2001. He abandoned the Republicans to become an independent in 2007.
Trump is frequently criticized by environmentalists for denying the potential dangers of climate change.
He frequently clashed with the city's labor unions, and presided over a spike in homelessness.
At the end of the speech, Bloomberg fell back on his track record as a businessman to criticize Trump.
"Most of us don’t pretend that we’re smart enough to make every big decision by ourselves," he said. "And most of us who have our names on the door know that we are only as good as our word, but not Donald Trump."