[1] Washtenaw County comprises the Ann Arbor Metropolitan Statistical Area.

The first peoples occupying the central portion of what is now Michigan included: "the Pottawattamies, the Chippewas, the Ottawas, the Wyandottes and the Hurons".

[5] Native Americans whose territories included land within the Washtenaw County boundaries are shown to have included: Myaamia (Miami),[6] Bodéwadmiké (Potawatomi),[7] Anishinabewaki ᐊᓂᔑᓈᐯᐗᑭ,[8] Peoria,[9] Meškwahki·aša·hina (Meskwaki),[10] and the Mississauga nation.

In the 1820s and 1830s, the events surrounding the independence of Greece from Turkey inspired construction of Greek Revival buildings, and the names of townships, towns, and children.

Following resolution of the Toledo War (1835–1836), in which Michigan Territory ceded its claim to Toledo in exchange for most of the Upper Peninsula (from the Porcupine Mountains eastward), Ohio withdrew its objection and Michigan became a state on January 26, 1837.

The convention also decided to move the capital from Detroit to a point further away from the Canadian border.

The University of Michigan, founded at Detroit in 1817, was moved to Ann Arbor in 1839 as a consolation for the city not being named the new state capital, as it had sought.

The name was changed in 1956 to Eastern Michigan College, which was elevated to university status in 1959.

The county government operates the jail, maintains rural roads (through a largely independent road commission), operates the major local courts, records deeds and mortgages, maintains vital records, administers public health regulations, and participates with the state in providing welfare and social services.

The county board of commissioners controls the budget and has limited authority to make laws or ordinances.

In Michigan, most local government functions – police and fire, building and zoning, tax assessment, street maintenance, etc.

Since 1988, when Michael Dukakis won it, the county has been a Democratic stronghold in local and national elections due to the presence of Ann Arbor and the University of Michigan.

In the 2020 United States presidential election it gave 72.4% of the vote to Democratic nominee Joe Biden, the highest margin for a Democrat in the county, the third-highest for any candidate in the county's history, and the highest in the state at the time as well.