[3] Washington County is part of the Omaha-Council Bluffs, NE-IA Metropolitan Statistical Area.

It was explored by Europeans as early as 1739 by Pierre Antoine and Paul Mallet,[5] who were on a trapping expedition to Canada.

In 1804, Lewis and Clark reported the establishment of the new United States government to a council of Indian chiefs near the present site of Fort Calhoun.

As a result of this council, Fort Atkinson was established in 1819 and served as a key midwestern outpost until 1827.

[7][8][9] The county seat has been in three different towns: Fort Calhoun, DeSoto, and Blair, its present site since 1869.

[10] The Fort Calhoun Nuclear Generating Station south of Blair, which was North America's smallest commercial nuclear reactor by rated capacity, was closed in October 2016 to begin decommissioning.

[11] An associated system of warning sirens was located in the southeastern part of the county for emergency notification in the event of a problem at the station.

Washington County contains Blair Municipal Airport, and several small privately owned grass airstrips, such as the Orum Aerodrome.

As of 2020, in only one presidential election since 1936, that being Lyndon Johnon's narrow victory in 1964, has the county selected the Democratic Party candidate, and even he was only able to win it by a single point.

President Donald Trump received 70% of the county's vote in the most recent presidential race, in 2024.