[2] The county was formed on January 13, 1846, and named after John Marshall, Chief Justice of the United States Supreme Court.

[3] Marshall County comprises the Marshalltown, Iowa Micropolitan Statistical Area.

In 2010, the center of population of Iowa was located in Marshall County, near Melbourne.

The population ranking of the following table is based on the 2020 census of Marshall County.

[1] † county seat Prior to 1964, Marshall County was strongly Republican, never backing a Democratic presidential candidate from 1880 to 1960 and only failing to back a Republican candidate during those years in 1912 with a strong third party campaign by former president Theodore Roosevelt on the Bull Moose ticket.

Since then, it has become far more of a swing county, having backed the national winner in the seven presidential elections from 1992 to 2016.

That streak was broken in 2020, when Donald Trump won the county but lost nationally.