It was named in honor of John Marshall, Chief Justice of the United States Supreme Court, who died in 1835.

[5] As of the 2010 United States census, there were 12,640 people, 5,161 households, and 3,549 families living in the county.

[12] In terms of ancestry, 43.0% were German, 16.7% were Irish, 14.4% were English, 7.2% were Italian, 6.2% were American, and 6.1% were Polish.

Its reputation as a swing county was to be sustained with the growth of the Republican Party: it voted for the winning candidate in every election from 1852 to 1912 except 1884 and 1888.

[citation needed] Since World War I, Marshall has generally been a strongly Republican county.