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.
House Speaker Mike Johnson put out a statement criticizing Mamdani and the Democrats after his historic win.
Johnson said Mamdani's win "cements" the Democratic Party's "transformation to a radical, big-government socialist party."
"Now, every House Democrat incumbent and candidate will co-own Mamdani’s disastrous record in the 2026 midterms," he added.
-ABC News' Lauren Peller
Mamdani posted a short video on Bluesky after several outlets projected he would win the New York City mayoral election.
The video showed a subway door opening at the City Hall stop, with an announcement saying, "The next stop is City Hall."
As of 9:30 p.m. with 63% of the expected vote reporting, Democrat Mikie Sherrill is projected to win the New Jersey gubernatorial election against Republican Jack Ciattarelli, currently holding a more than 14-point lead (57% to 42.4%).
"New Jersey, it is the honor of my life to earn your trust to become this great state's 57th Governor," Sherrill said in a post on X celebrating her victory. "I promise to listen, lead with courage, and never forget who I serve."
The former Navy pilot makes history as the first Democratic female governor of New Jersey. Former Republican Gov. Christine Todd Whitman, who served two terms from 1994 to 2001, was the state's first female governor.
Sherrill's victory breaks a notable trend in the state, as no party has controlled the New Jersey governor's seat for three consecutive terms since the 1960s. Her Democratic governorship will immediately succeed current Democratic Gov. Phil Murphy's back-to-back terms.
Tuesday’s results mark Ciattarelli’s third unsuccessful bid for governor. The former state assemblyman set his sights on the governor's mansion in 2017, but failed to secure the Republican primary. He made it to the general election in 2021, but lost to Murphy by just 3 points.
Sherrill consistently led Ciattarelli in the polls throughout the course of the campaign, though their distance narrowed slightly in the days leading up to the election.
Ciattarelli's loss comes as Republicans have tightened their margins in New Jersey in recent years, with Trump losing the state by just 6 points in 2024, compared to 16 points in 2020.
-ABC News' Emily Chang
ABC News projects Democrat Zohran Mamdani will win the New York City mayoral election.
He is set to be the city's first Muslim mayor of south Asian descent.
