Bloomberg, who has almost two years left in his second term at City Hall, had publicly denied any interest in running for president since one of his political advisers first planted the seed more than two years ago.
But his denials grew weaker in recent months as aides and supporters quietly began laying the groundwork for a third-party campaign.
"I listened carefully to those who encouraged me to run, but I am not - and will not be - a candidate for president," he wrote.
Among his biggest obstacles was getting on the ballot, a process that varies wildly from state to state and would have required him to obtain hundreds of thousands of signatures according to a timetable on which the first key date is March 5.
