As of the census of 2000, there were 53,597 people, 14,410 households, and 12,666 families residing in the county. The population density was 44 people per square mile (17/km²). There were 17,589 housing units at an average density of 14 per square mile (6/km²). The racial makeup of the county was 87.92% White, 0.15% Black or African American, 0.25% Native American, 0.28% Asian, 0.04% Pacific Islander, 9.91% from other races, and 1.46% from two or more races.
97.54% of the population were Hispanic or Latino of any race. According to the Census Bureau, Starr County had the highest percentage of Hispanic residents of any county in the United States.
There were 14,410 households out of which 54.70% had children under the age of 18 living with them, 66.50% were married couples living together, 17.40% had a female householder with no husband present, and 12.10% were non-families. 11.30% of all households were made up of individuals and 5.90% had someone living alone who was 65 years of age or older. The average household size was 3.69 and the average family size was 4.01.
In the county, the population was spread out with 37.40% under the age of 18, 11.00% from 18 to 24, 27.10% from 25 to 44, 16.30% from 45 to 64, and 8.20% who were 65 years of age or older. The median age was 26 years. For every 100 females there were 94.20 males. For every 100 females age 18 and over, there were 88.10 males.
The median income for a household in the county was $16,504, and the median income for a family was $17,556. Males had a median income of $17,398 versus $13,533 for females. The per capita income for the county was $7,069, which is the third-lowest in the United States. About 47.40% of families and 50.90% of the population were below the poverty line, including 59.40% of those under age 18 and 43.30% of those age 65 or over.