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.
More than half of voters in New York City said Tuesday that the cost of living was the most important issue facing the city, dwarfing the other issues measured in preliminary ABC News exit polling.
-ABC News' Emily Guskin
The economy is the biggest issue for Virginia voters, according to an exit poll analysis by ABC News.
Most Virginia voters said federal government cuts are affecting their finances. Most voters said Virginia’s economy is doing well, but few said their personal finances are improving.
Many voters in Virginia are employed or were previously employed by the federal government or as a federal contractor -- many of whom are currently furloughed.
-ABC News' Emily Guskin
An official at the Democratic Governors Association told ABC News that the group is encouraged by day-of turnout trends in New Jersey and Virginia, and say they believe the races breaking in their favor boil down to this central messaging difference.
"Biggest contrast in both races is that Dems hammered a message on the economy and never lost focus on affordability and their own positive plans to lower costs, while the GOP candidates refused to distance themselves from Trump even when he was doing a ton of damage to the local economy like DOGE in VA and the Gateway tunnel in NJ," the official said.
-ABC News' Brittany Shepherd
New York City's Board of Elections released new vote tallies as of 6 p.m., and they show the historic turnout continues to grow.
Over 1.7 million votes have been cast, according to the BOE. Of those votes, 1.01 million were cast Tuesday, according to the data.
Turnout has already surpassed a 20-year high.
