Spotswood stood at the confluence of the Manalapan and Matchaponix, which form the South River, and these waterways powered a series of industries beginning with a saw mill established by the 1720s by the noted regional Lenape leader Weequehela. By the 1730s, the Johnston brothers had taken over the site, and the settlement bore their name before it became Spotswood. During the American Revolution, local militia raided the mills while they were controlled by Loyalist owners and carried away large quantities of supplies, including iron and more than 5,000 pounds of meat, and the mills were later placed under militia guard to prevent similar British attacks. After the Revolution, the mill seat served as a saw mill, paper and grist mill, and by the 1850s, for snuff manufacturing, with operations ending by the 1920s.