The County of Warren was separated from Sussex County by an act of the Legislature
passed November 20, 1824. It was named for Dr. Joseph Warren, a Revolutionary War hero who
fell in the Battle of Bunker Hill. The seat of justice for the new county was permanently established in Belvidere on April 20, 1825.

The townships or civil divisions at the time of its organization were Greenwich, Hardwick, Independence, Knowlton, Mansfield, Oxford and Pahaquarry. These were represented in the first Board of Chosen Freeholders, which met at Belvidere on May 11, 1825.

The first Europeans to come into the County were the Dutch, who came to Pahaquarry Township and dug for copper in the early 1600s. Already here were many temporary and permanent camp sites of territorial tribes of American Indians who relied on hunting and agriculture for their existence. During this period, the Dutch constructed a road from Pahaquarry to Kingston, New York, over which they transported the proceeds of their mining ventures.

This road, the first commercial highway built in the United States, is known as "The Old Mine Road." At Oxford are the ruins of an iron ore blast furnace which provided cannon balls for the Revolutionary War. It was constructed in 1741 and, except for a twenty year period, was active until 1884 when it was finally blown out. This furnace was the first in America to use the hot blast method.