Fairfield Township was formed by Royal charter on May 12, 1697, from portions of the Cohansey Township area, while still part of Salem County, and was formed as a precinct in the newly created Cumberland County on January 19, 1748. It was incorporated on February 21, 1798, as one of the initial group of 104 townships in the state by an act of the New Jersey Legislature. Portions of the township were taken to form Downe Township (September 26, 1772), Millville Township (February 24, 1801) and Lawrence Township (February 17, 1885).
The name Fairfield refers to its original settlers, who were émigrés from Fairfield, Connecticut. The oldest Presbyterian church in South Jersey is said to date from before 1697. There is a plaque and an old cemetery which indicates the original log building.
According to the United States Census Bureau, the township had a total area of 43.950 square miles (113.830 km2), including 41.260 square miles (106.864 km2) of land and 2.690 square miles (6.966 km2) of water (6.12%).
Other unincorporated communities, localities and place names located partially or completely within the township include Back Neck, Ben Davis Point, Clarks Pond, Fordville, Gouldtown, Lanings Wharf, Mulfords Landing, New England Crossroads and Sea Breeze and Tindells Landing.