Douglas County is lightly wooded, mostly with ponderosa pine, with broken terrain characterized by mesas and small streams. Cherry Creek and Plum Creek rise in Douglas County and flow north toward Denver and into the South Platte River. Both were subject to flash flooding in the past, Plum Creek being partially responsible for the Denver flood of 1965. Cherry Creek is now dammed.
Residents generally commute to workplaces elsewhere in the metropolitan area outside of the county. Rapid sprawl is gradually displacing the ranching economy of the county.
As of the census of 2000, there were 175,766 people, 60,924 households, and 49,835 families residing in the county. The population density was 209 people per square mile (81/km²). There were 63,333 housing units at an average density of 75 per square mile (29/km²). The racial makeup of the county was 92.77% White, 0.95% Black or African American, 0.41% Native American, 2.51% Asian, 0.06% Pacific Islander, 1.43% from other races, and 1.88% from two or more races. 5.06% of the population were Hispanic or Latino of any race.
There were 60,924 households out of which 47.2% had children under the age of 18 living with them, 73.8% were married couples living together, 5.7% had a female householder with no husband present, and 18.2% were non-families. 13.3% of all households were made up of individuals and 1.9% had someone living alone who was 65 years of age or older. The average household size was 2.88 and the average family size was 3.19.
In the county the population was spread out with 31.6% under the age of 18, 4.8% from 18 to 24, 37.9% from 25 to 44, 21.6% from 45 to 64, and 4.2% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 99.7 males. For every 100 females age 18 and over, there were 97.4 males.
The median income for a household in the county was $82,929, and the median income for a family was $88,482 (these figures had risen to $93,819 and $102,767 respectively as of a 2007 estimate). Males had a median income of $60,729 versus $38,965 for females. The per capita income for the county was $34,848. About 1.6% of families and 2.1% of the population were below the poverty line, including 1.9% of those under age 18 and 3.7% of those age 65 or over.
Douglas County had the highest median household income of any Colorado county or statistical equivalent in 2000, as poverty is very low.
Currently, it is debated that a recent lite-rail system from downtown Denver to Douglas County has increased the apparent daytime homeless presence in some areas, but poverty rates remain very low and it is far from an issue.