"Nobody thought he had a shot at this," Bloomberg told CNBC's "Halftime Report" Tuesday. "Trump marches to his own drummer it would seem, and so far, that drummer has been playing the right tune to get him this far."
Only history will tell if that tune will carry him to the finish line, Bloomberg said. One of Trump's tunes has tapped into a common fear voters may have about the current and future state of the country.
"They're worried about losing their jobs; they're worried about what their kids are doing," Bloomberg told CNBC at a BTIG Charity event. "They are looking for something different."
The same approach, he said, has worked for Democratic candidate Bernie Sanders although he noted that the "bookies and polls" predict Hillary Clinton and Donald Trump will face off in the general election.
Bloomberg offered some advice for the nominees:
"The public wants simple solutions to complex problems," he said, adding that if there were actually simple solutions, these problems wouldn't exist at all. "But they also want to make sure that if you promise them, that you know what you're doing."
The candidates' main job before November will be to lay out tangible plans, Bloomberg said.
"It's nice to say we're going to create jobs; OK, how are you going to create jobs?" Bloomberg said. "It's nice to say we're going to make the world safer. OK, what are you gonna do?"
The billionaire businessman and philanthropist suggested voters sit back and really listen to decide who "they think can do that job."
"You want somebody that has experience; you want somebody that has judgment," Bloomberg said. "You want somebody that knows that they're not the smartest person in the world, and they will bring in experts."
The president doesn't need an answer to everything, Bloomberg said. But they do need the ability to attract the best advisors and cabinet members.
"Running the railroad is the way I would describe it, and that is the president's job," Bloomberg said. "That, and getting along with Congress."
The three-term mayor said he doesn't regret not running for president as an independent candidate. After "a lot of polling," Bloomberg concluded he would have gathered one third of the vote but would not have made it to the White House.
"You just can't win as a third-party candidate," Bloomberg said. He laid out a scenario in a three-person race, where no one candidate wins a majority and the House of Representatives is left to decide.
If the House were in Republican hands, for example, they would pick the Republican candidate. The same goes for a Democrat, leaving the independent with little hope at ending up in the Oval Office.
"We would get a third of the votes but we're not going to get a majority," he said, adding that you can spend your money and do a lot of good things in life besides public office. "Being the president isn't the only way to help people."