Complex sentence: The school made him rich, and he became one of the leading citizens of Poughkeepsie, serving as mayor from 1871--1874 and again from 1877 until his death.
Split into simpler sentences: The school made him rich, and he became one of the leading citizens of Poughkeepsie. He was Mayor from 1871 to 1874, and again from 1877 until his death.