Seawright, an Upper East Side mother of two, won 45.3 percent of the votes, with Gus Christensen earning 37.7 percent, David Menegon 10.1 percent and Ed Hartzog 7.8 percent.
She will now face Republican candidate David Garland in the Nov. 4 general election.
Seawright, a PTA mom, said public education is the biggest issue in the district, which runs from Third Avenue to the East River between 61st and 92nd streets and also includes Roosevelt Island.
She plans to fight for more middle schools, extra resources for teachers and smaller class sizes, she told DNAinfo New York in an interview in August.
“I’m a good negotiator,” she said in the August interview. “Chairing several boards has given me the negotiating skills to bring all sides together for a common goal.”