[a] On April 2, 1784,[a] Charlotte County was renamed Washington County in honor of George Washington, the American Revolutionary War general and later President of the United States of America.

[a][4] In 1994, with the completion of the new municipal center, the county seat was moved from Hudson Falls to Fort Edward.

Washington County has four historic covered bridges, each listed on the National Register of Historic Places: Including those, it has a total of 35 sites listed on the National Register.

The Lemuel Haynes House is designated as a National Historic Landmark, the highest level of significance.

Much of the county is part of the slate valley of the Upper Taconic Mountains (Taghkanic, meaning 'in the trees').

The eastern boundary of Washington County is the New York–Vermont border, part of which is Lake Champlain.

Western boundaries include primarily the Hudson River and Lake George.

Washington County belongs to the following valleys and watersheds: Champlain Valley / Lake George Watershed—02010001 [6] Hudson River Valley / Hudson-Hoosic Watershed—02020003 [6] Waters in the northern part drain into Lake Champlain via Lake George (Horican) or the Mettawee River, and then flow into the Saint Lawrence River (Kaniatarowanenneh).

17.5% were of Irish, 14.1% French, 12.1% English, 11.1% American, 9.0% Italian and 7.7% German ancestry according to Census 2000.

No Democrat aside from Lyndon B. Johnson in the aforementioned 1964 election has managed to win majority of the county's votes.

The following public use airports are located in the county:[17] Amtrak's Adirondack and Ethan Allen Express services each travel through Washington County once a day in each direction on their routes between New York, New York and Montreal, Québec or Burlington, Vermont, respectively.

Both routes stop in Fort Edward and the Adirondack additionally serves Whitehall.

The Adirondack was temporarily suspended from March 2020 through early April 2023 due to the closure of the Canadian/American border in response to the COVID-19 pandemic and related logistical challenges.