Thornhill is a home rule-class city in Jefferson County, Kentucky, United States.

Kentucky Route 22 (Brownsboro Road) forms the southern boundary of the city.

According to the United States Census Bureau, Thornhill has a total area of 0.05 square miles (0.12 km2), all land.

There were 75 housing units at an average density of 1,633.0 per square mile (630.5/km2).

The racial makeup of the city was 99.43% White and 0.57% Native American.