Hennepin is located at (41.256712, -89.329899).
According to the United States Census Bureau, the village has a total area of 5.6 square miles (14.5 km²), of which, 5.2 square miles (13.5 km²) of it is land and 0.4 square miles (0.9 km²) of it (6.45%) is water.
There were 304 households out of which 26.6% had children under the age of 18 living with them, 59.2% were married couples living together, 5.6% had a female householder with no husband present, and 32.2% were non-families. 30.3% of all households were made up of individuals and 16.1% had someone living alone who was 65 years of age or older. The average household size was 2.32 and the average family size was 2.87.
In the village the population was spread out with 20.8% under the age of 18, 9.2% from 18 to 24, 24.6% from 25 to 44, 27.0% from 45 to 64, and 18.4% who were 65 years of age or older. The median age was 42 years. For every 100 females there were 96.4 males. For every 100 females age 18 and over, there were 92.4 males.
The median income for a household in the village was $46,827, and the median income for a family was $56,111. Males had a median income of $48,500 versus $19,231 for females. The per capita income for the village was $23,981. About 2.6% of families and 2.9% of the population were below the poverty line, including 5.6% of those under age 18 and 2.9% of those age 65 or over.