[2] The county was named to honor William W. Hamilton, a President of the Iowa State Senate.

[12] As of the census[13] of 2000, there were 16,438 people, 6,692 households, and 4,597 families residing in the county.

The racial makeup of the county was 96.71% White, 0.23% Black or African American, 0.20% Native American, 1.46% Asian, 0.01% Pacific Islander, 0.61% from other races, and 0.78% from two or more races.

27.50% of all households were made up of individuals, and 13.60% had someone living alone who was 65 years of age or older.

Source:[15] The population ranking of the following table is based on the 2020 census of Hamilton County.

In the last 100 years, it has backed the losing candidate nationally only four times in the 1960, 1988, 2012 and 2020 presidential elections.