Born in Prussia, Wagner migrated with his family to the United States in 1885. After graduating from New York Law School, Wagner won election to the New York State Legislature, eventually becoming the Democratic leader of the state senate. Working closely with fellow New York City Democrat Al Smith, Wagner and Smith embraced reform, especially to the benefit of their core constituency, the working class. They built a coalition for these reforms that embraced unions, social workers, some businessmen, and numerous middle-class activists and civic reform organizations across the state. Wagner left the senate in 1918, and served as a justice of the New York Supreme Court until his election to the Senate in 1926.