The county was created in 1834 as part of Michigan Territory and organized the following year.

After the city of Milwaukee, the most populous in 2019 were West Allis (59,890), Wauwatosa (48,118), Greenfield (37,221), Oak Creek (36,325), and Franklin (35,811).

Portions of what is now Milwaukee County are known to have been inhabited by a number of Native American tribes, including the Sauk, Meskwaki or "Fox", Menomonee, Ojibwe and Potawotami, with elements of other tribes attested as well.

[5] In 1818, when the land later to be Wisconsin was made part of Michigan Territory, territorial governor Lewis Cass created Brown County, which at that time included all the land now part of Milwaukee County.

This territory encompassed all of what are now Milwaukee, Jefferson, Kenosha, Ozaukee, Racine, Rock, Walworth, Washington, and Waukesha counties, as well as large parts of the present-day Columbia, Dane and Dodge counties.

The racial makeup of the county (including Hispanics in the racial counts) was 52.0% White, 26.2% Black or African American, 4.9% Asian, 0.8% Native American, 6.8% from other races, and 9.3% from two or more races.

33.7% of all households were made up of individuals, and 10.1% had someone living alone who was 65 years of age or older.

33.0% of all households were made up of individuals, and 10.7% had someone living alone who was 65 years of age or older.

According to the U.S. Census Bureau, from 1980 to 2000, the residential pattern of Blacks versus Whites in Milwaukee County was the most segregated in the country.

[21] Additionally, there were 2,347 reported induced abortions performed on women of Milwaukee County residence, with a rate of 11.1 abortions per 1000 women aged 15–44, which is above the Wisconsin average rate of 5.2.

Former County Sheriff David Clarke, while repeatedly nominated and elected as a Democrat, was initially appointed by a Republican governor, stated that he considered himself nonpartisan, and espoused politically conservative positions.

In May 2019, the Milwaukee County executive became the first local government in the US to issue a declaration stating that racism constitutes a public health emergency.

The city of Milwaukee also operates The Hop tram system in the downtown area.