Lewis County was formed from Oneida, March 28, 1805. It was named in honor of Governor Morgan Lewis. It lies mostly within the valley of Black River, north of the center of the state. It is centrally distant 116 miles from Albany and contains 1,288 square miles.
A systematic settlement was first made under Seymour Green and Diadate Pease, agents of the Pierrepont Estate. In 1860, much of the region remained a wilderness.