Mamdani urged the city to "breathe this moment in."
Democrats triumphed in high-profile races that unfolded across the country on Tuesday, including New York City's mayoral election and governor's races in New Jersey and Virginia.
In California, voters also chose to redraw their state's congressional map in a move that could net Democrats five House seats.
The elections help provide a picture of how Americans feel about President Donald Trump's first nine months in office. Following a wave of Republican losses, Trump tried to blame the election results on his not being on the ballot and the prolonged government shutdown.
Senate Minority Leader Chuck Schumer dodged a question by a reporter who asked if he voted for Mamdani or Cuomo in the election.
"Look I voted, and I Iook forward to working with the next mayor to help New York City," he said before taking another question.
Schumer has not publicly supported Mamdani since he won the Democratic primary in June.
-ABC News' Lauren Peller
As of 3 p.m. ET, roughly 1.4 million New York voters have cast a ballot in this year's elections, with six hours before polls close marking the highest turnout for a city election in 20 years, the city's Board of Elections said.
Roughly 1.1 million voters cast a ballot in the 2021 election, according to city BOE data.
The last time a city election vote exceeded 1.3 million was in 2005, when Michael Bloomberg was re-elected to a second term, according to BOE records.
Of the 1.4 million cast so far in 2025, 716,625 votes, about 49%, were cast on Tuesday, while the remaining were cast at early voting polling sites.
The California secretary of state slammed President Donald Trump over his unfounded claims about the state's election.
Secretary of State Shirley Weber said in a statement that Trump's comments appeared to be "another baseless claim"-- and urged voters to head to the polls.
"California voters will not be deceived by someone who consistently makes desperate, unsubstantiated attempts to dissuade Americans from participating in our democracy," she said.
Additionally, a spokesperson for the state's attorney general said that Trump is "continuing to spread lies," adding that elections in California are "fair, safe, and secure."
-ABC News' Olivia Rubin
After President Donald Trump wrote in a social media post that California's Proposition 50 should be under "very serious legal and criminal review," White House press secretary Karoline Leavitt said the White House is "looking into" providing "executive action."
"The White House is working on an executive order to strengthen our elections in this country, and to ensure that there cannot be blatant fraud, as we've seen in California with their universal mail-in voting system. It's absolutely true that there's fraud in California elections. It's just a fact," Leavitt told reporters on Tuesday.
Leavitt's comments come after Trump laid into the proposal -- where Californians are deciding if the state will adopt a new Democratic-friendly congressional map in response to mid-decade redistricting in Texas -- calling it a "GIANT SCAM."
"The Unconstitutional Redistricting Vote in California is a GIANT SCAM in that the entire process, in particular the Voting itself, is RIGGED. All 'Mail-In' Ballots, where the Republicans in that State are 'Shut Out,' is under very serious legal and criminal review. STAY TUNED!" Trump wrote in a social media post on Tuesday.
