According to the United States Census Bureau, the borough has a total area of 2.7 square miles (7.0 km²), of which, 2.6 square miles (6.7 km²) of it is land and 0.1 square miles (0.3 km²) of it (3.70%) is water. Northampton's elevation is above sea level.
The Lehigh River provides the western border of the borough.
There were 3,869 households out of which 30.3% had children under the age of 18 living with them, 53.3% were married couples living together, 10.3% had a female householder with no husband present, and 31.9% were non-families. 27.4% of all households were made up of individuals and 14.3% had someone living alone who was 65 years of age or older. The average household size was 2.40 and the average family size was 2.92.
In the borough the population was spread out with 22.4% under the age of 18, 5.8% from 18 to 24, 32.2% from 25 to 44, 21.0% from 45 to 64, and 18.6% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 91.4 males. For every 100 females age 18 and over, there were 86.1 males.
The median income for a household in the borough was $40,982, and the median income for a family was $49,482. Males had a median income of $37,033 versus $26,422 for females. The per capita income for the borough was $19,516. About 1.3% of families and 3.7% of the population were below the poverty line, including 1.4% of those under age 18 and 5.9% of those age 65 or over.
Tru Blu brewery, a microbrewery, is based in Northampton.
Northampton was created from the villages of Siegfried, Newport and Stemton, which together were formed into an alliance in 1902. In 1909, Atlas petitioned county courts to change the alliance into a borough, reportedly because the change would make it easier for the company to send and receive mail.
The Atlas Portland Cement Company plant closed in 1982, but borough residents — many of them former Atlas employees or their descendants — still identify strongly with the company and its history.
The Atlas Portland Cement Company supplied most of the cement used in the building of the Panama Canal. The residents of Northampton are very proud of their town's help in building one of the most famous canals in the history of the world.