The estimate by two polling companies saw Hidalgo, with between 49.3 and 50.2 percent of the vote, ahead of right-wing candidate Rachida Dati, with President Emmanuel Macron's candidate a distant third.
Speaking on Sunday night Hidalgo thanked all those who had voted for her
“You chose hope, you chose unity, you chose a Paris that breathes, a Paris that will be more pleasant to live in, a more united city, which leaves no one on the side of the road,” she said.
Analysts expect the election to confirm that Macron's centrist Republic on the Move (LREM) party — founded by the president ahead of his 2017 election win — has failed to gain a strong foothold at local level.
The party made lacklustre showings in March — notably in Paris where Macron's candidate, former health minister Agnes Buzyn, came third.
“The problem is that the LREM is a new party that has no local roots and is struggling to impose itself as a (political) force,” analyst Jean Garrigues of the University of Orleans told AFP.
Amid persistent fears of coronavirus contagion, just over a third of voters had turned out by 5:00 pm, three hours before polling stations close, the interior ministry said.
The turnout rate of 34.67 percent was lower even than nine hours into the first round of voting on March 15 that was marked by a record 55-percent abstention rate.
She will probably double down on her crackdown on apartment rentals.