Bill de Blasio said the relevant municipal agencies had backed the idea.
The bronze Fearless Girl statue was placed on 8 March to draw attention to gender inequality and the pay gap in the corporate world.
The statue - which has become an instant tourist draw - had been due to be removed on Sunday.
Appearing next to the 4ft-tall (1.21m) statue in Manhattan on Monday, Mr de Blazio said it "means so much to the people of New York".
The statue was "standing up to fear, standing up to power, being able to find in yourself the strength to do what's right", he added.
"She is inspiring everyone at a moment when we need inspiration."
The work, by artist Kristen Visbal, was commissioned by asset managers State Street Global Advisors (SSGA).
The company said the girl represented the future.
It added that one in four of the 3,000 largest traded US companies did not have even one woman on their board.
Wall Street's Charging Bull statue was originally guerrilla art by Italian-born artist Arturo Di Modica.
Installed in 1989, the bronze sculpture was meant to represent the "strength and power of the American people" in response to the market crash in 1987.
But it become a popular attraction and was allowed to stay.