Born in Shawangunk (now Wallkill), Ulster County, New York, he received a limited schooling, became a surveyor and was engaged in agricultural pursuits. He owned slaves.^[1] During the American Revolutionary War, he was a member of the committees of vigilance and safety. He was a member of the New York State Assembly (Ulster Co.) from 1777 to 1790 and was a member of the State Convention to ratify the U.S. Constitution in 1788. In April, 1790, Schoonmaker was elected to the 2nd United States Congress, holding office from March 4, 1791, to March 3, 1793. He was again a member of the State assembly in 1795.

Schoonmaker died in Shawangunk in February, 1796, and was interred in Old Shawangunk Churchyard at Bruynswick, in Shawangunk.

Congressman Marius Schoonmaker (1811–1894) was his grandson.

• United States Congress. "Cornelius C. Schoonmaker (id: S000140)". Biographical Directory of the United States Congress.

• "Schoonmaker, Cornelius" . Appletons' Cyclopædia of American Biography. 1900.