As of the census of 2000, there are 20,814 people in the county, organized into 8,521 households, and 6,083 families. The population density is 20 people per square mile (8/km²). There are 9,902 housing units at an average density of 9 per square mile (4/km²). The racial makeup of the county is 92.82% White, 0.33% Native American, 0.21% Black or African American, 0.18% Asian, 0.02% Pacific Islander, 5.27% from other races, and 1.18% from two or more races. 15.90% of the population are Hispanic or Latino of any race. In 1990 there were approximately 3,000 speakers of Texas German in Gillespie and Kendall counties, but this is believed to have declined in the last two decades.
There are 8,521 households out of which 25.90% have children under the age of 18 living with them, 62.10% are married couples living together, 7.00% have a female householder with no husband present, and 28.60% are non-families. 25.80% of all households are made up of individuals and 14.20% have someone living alone who is 65 years of age or older. The average household size is 2.38 and the average family size is 2.84.
In the county, the population is spread out with 21.60% under the age of 18, 5.50% from 18 to 24, 21.20% from 25 to 44, 26.20% from 45 to 64, and 25.50% who are 65 years of age or older. The median age is 46 years. For every 100 females there are 89.70 males. For every 100 females age 18 and over, there are 88.10 males.
The median income for a household in the county is $38,109, and the median income for a family is $45,315. Males have a median income of $26,675 versus $20,918 for females. The per capita income for the county is $20,423. 10.20% of the population and 7.10% of families are below the poverty line. Out of the total population, 13.40% of those under the age of 18 and 9.90% of those 65 and older are living below the poverty line.