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.
Democrat Ghazala Hashmi is the first Muslim woman to win statewide office in U.S. history.
The Council on American-Islamic Relations (CAIR) highlighted the historic moment.
"We hope this historic moment will inspire American Muslims to continue pursuing public service in Virginia and across the country," the organization said in a statement.
-ABC News' Brittany Shepherd
Following Spanberger's victory in Virginia, Trump's political adviser and 2024 co-campaign manager, Chris LaCivita, seemed to imply that Republican gubernatorial nominee Winsome Earle-Sears was a bad candidate.
"A Bad candidate and Bad campaign have consequences - the Virginia Governor's race is example number 1," LaCivita posted on X.
LaCivita's post comes after a tele-rally Monday night for Virginia's Republican candidates, during which Trump did not mention Earle-Sears by name.
-ABC News' Hannah Demissie
As of 9:01 p.m., with 41% of the expected vote in, Democrat Mikie Sherrill continues to lead against Republican Jack Ciattarelli by just over 14 points (56.7% to 42.4%) in the New Jersey governor's race.
This comes after a judge ruled for a one-hour extension of some Passaic County polling centers due to unfounded bomb threats earlier Tuesday. As of 9 p.m., all New Jersey polls are now closed.
While surrounded by her daughters during her victory speech, Abigail Spanberger remarked on the historic nature of her projected win in Virginia.
She said ahead of her speech that her husband said to their daughters, "Your mom's going to be the governor of Virginia."
"I can guarantee those words have never been spoken in Virginia ever before," she said.
"It's a big deal that the girls and the young women I have met along the campaign trail now know with certainty that they can achieve anything," she continued.
