Walpack Township dates back to October 26, 1731, when it was first mentioned as "Walpake" in Hunterdon County. The area covered by the present-day township was set off to Morris County upon that county's creation in 1739, and became part of the newly formed Sussex County in 1753. As of April 15, 1754, Walpack's boundaries were defined as a "precinct". Walpack was formally incorporated as a township by an act of the New Jersey Legislature on February 21, 1798. Portions of the township were taken to form Montague Township (March 26, 1759), Sandyston Township (February 26, 1762) and the now-defunct Pahaquarry Township in Warren County (December 27, 1824). Territory was gained from Stillwater Township in 1935.