According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. The county's northern border with Ohio is formed by the Ohio River.
As of the census of 2000, there were 16,800 people, 6,847 households, and 4,697 families residing in the county. The population density was. There were 7,754 housing units at an average density of. The racial makeup of the county was 90.88% White, 7.16% Black or African American, 0.15% Native American, 0.37% Asian, 0.02% Pacific Islander, 0.57% from other races, and 0.85% from two or more races. 0.95% of the population were Hispanic or Latino of any race. There were 6,847 households out of which 31.30% had children under the age of 18 living with them, 54.20% were married couples living together, 11.10% had a female householder with no husband present, and 31.40% were non-families. 27.60% of all households were made up of individuals and 12.80% had someone living alone who was 65 years of age or older. The average household size was 2.41 and the average family size was 2.92. In the county, the population was spread out with 24.10% under the age of 18, 8.00% from 18 to 24, 28.50% from 25 to 44, 23.90% from 45 to 64, and 15.50% who were 65 years of age or older. The median age was 38 years. For every 100 females, there were 93.70 males. For every 100 females age 18 and over, there were 89.50 males. The median income for a household in the county was $30,195, and the median income for a family was $37,257. Males had a median income of $30,718 versus $21,216 for females. The per capita income for the county was $16,589. About 12.90% of families and 16.80% of the population were below the poverty line, including 23.60% of those under age 18 and 13.70% of those age 65 or over.
Mason County was at the time of the Civil War the easternmost of the strongly secessionist Bluegrass bloc. Mason was in fact the most easterly Kentucky county to be represented at the Russellville Convention of 1861 to discuss the secession of Kentucky from the Union. Mason County's secessionist sentiment meant that it voted Democratic consistently up until the 1950s, with the exception of the 1928 election when strong local anti-Catholic sentiment against Al Smith allowed Herbert Hoover to carry the county. Since the 1950s, the state has turned Republican due to the increasingly liberal position of the Democratic Party on social issues.
