It was late in the afternoon on Tuesday, and Bilquees Akhtar was still at work as an assistant to the principal of EPIC High School North in Richmond Hill, Queens. Suddenly her phone exploded with text messages and DMs on Instagram and TikTok from her five adult children. Each of them had already cast a vote for Zohran Mamdani in New York City’s Democratic mayoral primary.

“MOM, WHY ARE YOU STILL AT WORK?” Ms. Akhtar’s 24-year-old son, Humza Mehfuz, wrote to her. “YOU HAVE TO VOTE!”

While Ms. Akhtar had previously supported Mr. Mamdani’s main opponent, Andrew M. Cuomo, when he ran for governor and, years before that, had voted for his father, former Gov. Mario Cuomo, she told her children to calm down. After their relentless campaign of showing her TikTok videos of Mr. Mamdani — “This kid is brilliant,” she had to admit, “and so friendly!” — she had made her decision.

“All of Cuomo’s ads tried to make Mamdani look like a terrorist,” said Ms. Akhtar, 56. “But he’s a New Yorker like me.”

By Wednesday, Mr. Mamdani, a state assemblyman from Queens, had won 43 percent of votes counted, all but clinching perhaps the greatest political upset in New York City politics in a generation. (The final tally is not expected to be completed until next week, but Mr. Cuomo conceded the race on Tuesday night.) If Mr. Mamdani were to win the general election this fall, he would be the first Muslim mayor in the history of New York, and also the first mayor of South Asian descent.