|Area||3.8 sq mi (10 km2)|
|- land||3.0 sq mi (8 km2)|
|- water||0.7 sq mi (2 km2)|
|Density||207.9 / sq mi (80 / km2)|
|- summer (DST)||EDT (UTC-5)|
Clark is located at .(41.282049, -80.409591)
According to the United States Census Bureau, the borough has a total area of 3.8 square miles (9.8 km2), of which, 3.0 square miles (7.8 km2) of it is land and 0.7 square miles (1.8 km2) of it (19.31%) is water.
As of the census of 2000, there were 633 people, 227 households, and 194 families residing in the borough. The population density was 207.9 people per square mile (80.1/km²). There were 233 housing units at an average density of 76.5 per square mile (29.5/km²). The racial makeup of the borough was 99.53% White, 0.16% African American, 0.16% from other races, and 0.16% from two or more races. Hispanic or Latino of any race were 0.47% of the population.
There were 227 households out of which 32.2% had children under the age of 18 living with them, 76.7% were married couples living together, 7.0% had a female householder with no husband present, and 14.1% were non-families. 11.9% of all households were made up of individuals and 5.3% had someone living alone who was 65 years of age or older. The average household size was 2.79 and the average family size was 3.02.
In the borough the population was spread out with 23.4% under the age of 18, 6.3% from 18 to 24, 28.0% from 25 to 44, 29.2% from 45 to 64, and 13.1% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 92.4 males. For every 100 females age 18 and over, there were 93.2 males.
The median income for a household in the borough was $53,438, and the median income for a family was $56,944. Males had a median income of $40,729 versus $26,625 for females. The per capita income for the borough was $24,892. About 1.0% of families and 1.7% of the population were below the poverty line, including none of those under age 18 and 4.3% of those age 65 or over.
Clark, Pennsylvania used to be called Clarksville back in the late 1950s. Because of flooding of the Shenango River a dam was built in Sharpsville, Pennsylvania and the borough of Clarksville was moved to higher ground that was farmland and forest. Houses were moved as well as a cemetery. Other houses that were not moved were used by the workers of the reservoir. They were then demolished. The Erie Canal extension went along this Shenango River route and is now under water. Clarksville had quite a history at one time with the Erie Canal coming through it. The old town is now covered by water, grass, trees and weeds. The only remains of it are some old asphalt streets that can be found amongst the trees, along with the main road, which is visible when the water is lowered. There is now a marina there that sits where businesses and residents used to be.