Even when he had a big city to run, the billionaire remained intimately involved in the affairs of his privately owned data-and-media powerhouse, as Fortune’s coverage of Bloomberg LP last year made clear.

On several occasions, the then-mayor showed up at Bloomberg headquarters after hours for business meetings—including a presentation on a redesign of Bloomberg’s website held during a blizzard. He monitored the company from his Bloomberg terminal at City Hall. And he received weekly briefings from Dan Doctoroff, his handpicked CEO.

So despite years of public declarations by both Bloomberg and Doctoroff that it would never happen, it hardly comes as a shock that Mike (as he is known at Bloomberg LP) is reassuming command of his business at the end of 2014, just under a year after leaving office. His previous plan had been to devote the majority of his time to his extensive philanthropic activities.

The odd man out, of course, is Doctoroff, 56, the former deputy mayor who has been running Bloomberg LP since 2008. That wasn’t an easy task. Even while Mike—who owns more than 80% of the business—was at City Hall, he loomed over everything at Bloomberg LP. “People live there in the shadow of Mike,” former HR chief Melinda Wolfe told Fortune last year. “There’s a constant questioning: What would Mike do?”

That reality—along with the presence of powerful sacred cows inside the company, such as news chief Matt Winkler and terminal boss Tom Secunda—created a management nightmare for Doctoroff. He labored to change Bloomberg’s eccentric, Mike-shaped culture, starting new businesses and reining in costs in the face of changing times and slowing terminal sales.

Bloomberg remains highly profitable and kept growing, with revenues expected to reach $9 billion for 2014. But with Mike back on the premises—and constitutionally unable to play the role of second banana—the situation quickly became untenable.

“Mike is kind of like God at the company,” Doctoroff explained on Wednesday, sitting with Bloomberg at a New York coffee shop, to Andrew Ross Sorkin of the New York Times. “He created the universe. He issued the Ten Commandments and then he disappeared. And then he came back. You have to understand that when God comes back, things are going to be different. When God reappeared, people defer.” (Through a spokesman, Bloomberg and Doctoroff both declined interviews.)

So what does this reappearance mean for the business—and the world?

Bloomberg LP’s lavish profits are driven by subscriptions to its financial-data terminals—the count is now 321,000—which cost $21,000 a year apiece. But its operations also include a huge news service, Bloomberg BusinessWeek, and a TV financial-news operation—all big money-losers.

Doctoroff had moved to diversify the company, starting new businesses and installing traditional corporate management systems and controls. Yesterday, Bloomberg told the Times he didn’t expect to shift direction. “The plan that we have is the plan we should execute,” he said.

But it’s hard to imagine that the new boss (same as the old boss) reclaimed his post simply to stay the course. In his previous incarnation as CEO, Bloomberg focused almost single-mindedly on increasing the number of terminals sold, trusted his gut, hated meetings, never laid anyone off, and didn’t worry about spending.

Mike, who now occupies a fifth-floor desk at the company’s lavish Lexington Avenue headquarters, has returned to a business three times the size of the one he last ran in 2001, with 16,000 employees. It’s hard to manage exactly the same way, and Bloomberg faces fresh challenges to its lucrative terminal franchise.

But, at 72, his management approach—and passions—seem unlikely to change. As mayor, he launched a string of crusades. Upon leaving office, he declared his intention to focus on philanthropy—Forbes places his wealth at $33 billion—as a way to continue his public-policy activism, which includes supporting gun-control and anti-smoking efforts.

Now that he’s again running Bloomberg LP, with an array of powerful platforms at his disposal, expect Mike Bloomberg to remain an unconventional CEO, focused far more on amplifying his voice and boosting his influence than on the mundane business of making money.