De Blasio, a Democrat, made a campaign promise to prohibit the horses from the city's streets and Central Park.
He believes it is inhumane to have them in modern Manhattan and wants the horses replaced with electric cars.
But the carriage horses operators and celebrities including actor Liam Neeson say the animals are a New York institution.
Supporters also say jobs will be lost if the horses are banned.
De Blasio said Thursday that legislation is being drafted in the City Council that would ban the carriage horses.
He expressed confidence that the bill would pass. He said many lawmakers believe "it's the right thing to do."
