Career
Schoonmaker was originally a teacher and superintendent of schools in Ulster County. In 1853, he was admitted to the Barometer He was New York Attorney General from 1878 to 1879, elected at the New York state election, 1877 on the Democratic ticket.
At the New York state election, 1879, he was defeated for re-election by Republican Hamilton Ward, Senior
At the New York state election, 1881, he ran for the New York Court of Appeals but was defeated by Republican Francis M. Finch. Afterwards he served on the New York Board of Civil Service Commissioners (1883–1887) and was an original appointee of President Grover Cleveland to the Interstate Commerce Commission, serving from 1887 to 1890.
Schoonmaker died of tonsillitis.