The City of Sunbury is the largest population center. The other main population centers in Northumberland County are Shamokin, Mount Carmel, Trevorton, Riverside, and Milton.

Northumberland County can be subdivided into three areas each with its own unique character. The northern most portion of the County is centered around Interstate 80. While the northern most portion was more historically agricultural based, the area now is characterized by light industrial uses that have located in the area to take advantage of proximity to Interstate 80. Access to Interstate 80 also provides residents of the northern most portion of Northumberland County with convenient access to employment centers in Columbia, Lycoming, Snyder, and Union Counties. Subsequently, the northern most portion of Northumberland County is the developing area of the County with new residential development and commercial uses to support the households. The development is generally suburban style low rise low density development. The central portion of the County, which is centered around the City of Sunbury, is situated adjacent to the Susquehanna River and is renowned for its fertile farmlands and forests. Outside of the settled Cities and Boroughs, the land use in the north-west portion of the County was recently developed.

Many of the communities in the central portion of Northumberland County contain single-family detached housing on large lots. Like the northern most portion of the County, the central portion of the County has good access to the regional road network, which provides convenient access to employment centers in Columbia, Lycoming, Snyder, and Union Counties. The access to employment has attracted the younger households residing in Northumberland to the north-west and central portions of the County.

The south east portion of Northumberland County is centered in around the City of Shamokin and Coal Township. Historically employment opportunities in the south east portion of Northumberland County were in coal mining and manufacturing with the manufacturing base heavily concentrated in the apparel and textile industries. The industrial uses were located among the residential uses and the mines encroached on the communities. Housing in the south east portion of Northumberland County was often developed by employers as accommodations for the employees.