|• Total||0.0 sq mi (0.1 km2)|
|• Land||0.0 sq mi (0.1 km2)|
|• Water||0.0 sq mi (0.0 km2)|
|Elevation||571 ft (174 m)|
|• Density||3,810.5/sq mi (1,471.2/km2)|
|Time zone||Eastern (EST) (UTC-5)|
|• Summer (DST)||EDT (UTC-4)|
|GNIS feature ID||1669525|
Thornhill is located at .
According to the United States Census Bureau, the city has a total area of 0.1 square miles (0.26 km2), all of it land.
As of the census of 2000, there were 175 people, 72 households, and 60 families residing in the city. The population density was 3,810.4 people per square mile (1,351.4/km²). There were 75 housing units at an average density of 1,633.0 per square mile (579.2/km²). The racial makeup of the city was 99.43% White and 0.57% Native American.
There were 72 households out of which 29.2% had children under the age of 18 living with them, 73.6% were married couples living together, 8.3% had a female householder with no husband present, and 15.3% were non-families. 13.9% of all households were made up of individuals and 6.9% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 2.66.
In the city the population was spread out with 21.1% under the age of 18, 2.9% from 18 to 24, 20.6% from 25 to 44, 31.4% from 45 to 64, and 24.0% who were 65 years of age or older. The median age was 47 years. For every 100 females there were 92.3 males. For every 100 females age 18 and over, there were 91.7 males.
The median income for a household in the city was $84,326, and the median income for a family was $100,391. Males had a median income of $61,000 versus $48,125 for females. The per capita income for the city was $38,698. None of the population or families were below the poverty line.