When he first declared his candidacy for mayor last fall, Assemblyman Zohran Mamdani was a state legislator with a thin résumé who was unknown to most New Yorkers.

Months later, he appears poised to become the Democratic Party’s nominee for mayor, having bested a far better known and more experienced cast of candidates who had deep relationships with voters across New York City.

Mr. Mamdani’s campaign focused intensely on the plight of working-class New Yorkers who were struggling with New York City’s affordability crisis, most notably the skyrocketing costs of housing and child care.

Here is a look at his record and some important things to know about New York City’s Democratic mayoral nominee:

Mr. Mamdani beat a four-term incumbent in a close State Assembly primary in 2020. He joined a small group of lawmakers in Albany who were part of the Democratic Socialists of America’s New York chapter. His agenda in Albany mirrored his campaign priorities, but of the 20-odd bills Mr. Mamdani has introduced in more than four years in Albany, just three relatively minor items have become law.

During the campaign, he talked extensively about a program to begin making city buses free that he had helped start. The pilot program lasted one year and was not renewed. Still, colleagues said his ideas had helped to move the ideological center of the Assembly to the left.

In Albany, he was one of the Legislature’s youngest members. If elected mayor, he would be, at 34, the city’s youngest leader since 1917, when John Purroy Mitchel, a reformer known as the “Boy Mayor,” was elected and served one term. Mr. Mamdani’s youth and fresh vision attracted a broad swath of progressive voters, even as his opponents focused on his relative lack of experience.

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