As of the census of 2000, there were 126,638 people, 44,725 households, and 35,780 families residing in the county. The population density was 217 per square mile (84 /km). There were 47,005 housing units at an average density of 81 per square mile (31 /km). The racial makeup of the county was 91.55% White, 5.18% Black or African American, 0.20% Native American, 1.25% Asian, 0.03% Pacific Islander, 0.97% from other races, and 0.82% from two or more races. 2.52% of the population were Hispanics or Latinos of any race.
By 2005 the racial composition of the county was 88.8% non-Hispanic white, 5.0% African-American, 3.4% Latino and 2.7% Asian-American.
There were 44,725 households in 2000 out of which 43.00% had children under the age of 18 living with them, 69.80% were married couples living together, 7.80% had a female householder with no husband present, and 20.00% were non-families. 16.60% of all households were made up of individuals and 4.50% had someone living alone who was 65 years of age or older. The average household size was 2.81 and the average family size was 3.18.
The age distribution was 29.50% under the age of 18, 6.20% from 18 to 24, 31.60% from 25 to 44, 24.90% from 45 to 64, and 7.70% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 97.00 males. For every 100 females age 18 and over, there were 93.70 males.
In 2007, the median income for a household in the county was $84,205, and the median income for a family was $97,688. In 2005, the per capita income for the county was $39,150. About 3.50% of families and 4.70% of the population were below the poverty line, including 5.40% of those under age 18 and 8.90% of those age 65 or over.
Williamson County is ranked among the wealthiest counties in the country. In 2002 a ranking of counties based on their median household income placed it thirteenth on the list of wealthiest counties. In 2006 it was the eleventh wealthiest county in the country, but the Council for Community and Economic Research ranked Williamson County as America's wealthiest county (1st) when the local cost of living was factored into the equation with median household income.
By 2006 Williamson County had a population of 160,781 representing 27.0% population growth since 2000. The census bureau lists Williamson as one of the 100 fastest growing counties in the United States for the period 2000-2005.
In the 2004 presidential election, Williamson County voted 72 percent in favor of George W. Bush, 27 percent in favor of Senator John Kerry, and 1 percent in favor of Ralph Nader.