1721 --- Civil administration as part of French Louisiana.

1763 --- British sovereignty, part of Western Wilderness Territory, military rule.

1774 --- British military rule, Province of Quebec.

1778 --- July 4 --- Proclaimed part of Illinois Country of Virginia, civil and military authorities appointed by Gov. Patrick Henery.

1773 --- Ceded by Virginia to the thirteen colonies in common, still unorganized territory. NOTE: (this year, 1773, was copied from a source document and it is possible that it should be 1783.)

1787 --- Part of St. Clair County, Northwest Territory of the Confederated American States, Arthur St. Clair, Governor.

1795 --- Mostly part of Randolph County, a small part of St. Clair County, Indiana Territory, U.S.A.

1800 --- Part of St. Clair County, Illinois Territory.

1818 --- Made a separate county of State (Washington), including all of Clinton County, Illinois.

1824 --- December 23 --- Clinton County separated, and Washington County began its existence in its present form.