The county was formed by the Michigan Legislature in 1840 as Mikenauk County, (from Ojibwe mikinaak meaning turtle) then renamed Roscommon County in 1843. It was administered by Michilimackinac (Mackinac), Cheboygan, and Midland counties, in succession, prior to the organization of county government in 1875. Roscommon County was named after County Roscommon, Ireland.
There are three official Michigan historical markers in the county: