[3] It is named from the Isle of Guernsey in the English Channel, from which many of the county's early settlers emigrated.

Guernsey County, located in the Appalachian foothills, was first formed and organized on March 10, 1810, from portions of Muskingum and Belmont counties, after the Ohio Legislature acted.

It lost some land area during the formation of neighboring counties until it reached its present boundaries in 1851, after Buffalo, Beaver, Olive, and Seneca townships were gained by Noble county.

[6] As of the census[11] of 2000, there were 40,792 people, 16,094 households, and 11,233 families living in the county.

26.10% of all households were made up of individuals, and 11.20% had someone living alone who was 65 years of age or older.

As of the 2010 United States Census, there were 40,087 people, 16,210 households, and 10,949 families living in the county.

[12] In terms of ancestry, 22.3% were German, 15.2% were Irish, 12.4% were English, and 9.6% were American.

The most recent of these Democrats to win the county was Bill Clinton in 1996.