Bloomberg owns 72% of the financial news company he set up more than 20 years ago. He gave up the day-to-day running of the business earlier this year to concentrate on his election campaign. He has now confirmed that he will not return.
The mayoral elections takes place on November 6. He said that if he loses, he will look for another political or public service role.
Bloomberg's personal fortune is estimated at more than $4bn (£2.8bn) and he has drawn criticism because he has so far spent $8m (£5.6m) of his own money on his campaign -- a figure that his rivals cannot match.