Gregoire's victory deepens leftist control of the French capital, where the Socialists have been in power for 25 years, overseeing an ecological urban renewal project that resonated with many voters.
Grégoire claimed victory on Sunday after estimates based on partial results placed him well ahead of Dati, who acknowledged defeat.
"Tonight is the victory of a certain vision of Paris: a vibrant Paris, a progressive Paris," Grégoire said.
French voters returned to the polls Sunday for the final round of municipal elections in 1500 communes, including major cities.
The vote is a test of the balance of power on France's local political map before the 2027 presidential race begins to take shape.
It is also a measure of whether the far right can convert national momentum into control of major cities, where it has often struggled to break through.
The most closely watched contests are concentrated in major cities after a first round that left France's traditional left and right competitive, the far right strongly placed in several urban races especially in southeastern France, and President Emmanuel Macron 's centrist movement keeping a low profile in many of the most closely watched races.
France's far-right National Rally (RN) fell short of taking control of the southern cities of Marseille and Toulon, exit polls showed, in municipal elections that gave hope to embattled mainstream parties.
Senior RN officials rejected suggestions that the party's defeat in Toulon showed it had hit a "glass ceiling" ahead of next year's presidential election, saying it had won dozens of local constituencies where it previously had no presence.
"The National Rally and its candidates have achieved tonight, in this municipal election, the biggest breakthrough in its entire history," RN chief Jordan Bardella said.
In the first round, his anti-immigration party won re-election in the southern city of Perpignan and won smaller cities too.
Eric Ciotti, a former mainstream conservative who is now an ally of the RN, won in Nice, France's fifth-biggest city, exit polls showed.
With wins projected in France's two biggest cities, Paris and Marseille, the Socialist Party, which is weakened nationally, saw reasons to hope.
"Only the left can prevent France from this regression," Socialist Party chief Olivier Faure said.
with AP
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