Greenwich Township has a long history passing through as part of most of Northwestern New Jersey's counties. The township was first mentioned in official documents on October 9, 1738, as a part of Hunterdon County. On March 15, 1739, it became part of the newly-formed Morris County. On January 22, 1750, portions of the township were taken to form Hardwick Township.
On June 8, 1753, Sussex County was created, and Greenwich Township was shifted again. Portions of the township were taken on May 30, 1754, to form both Mansfield Township and Oxford Township. The township was incorporated by an Act of the New Jersey Legislature on February 21, 1798
It found its current home when Warren County was formed on November 20, 1824. On April 8, 1839, portions of the township were taken to create Franklin Township and Harmony Township. On March 7, 1851, Phillipsburg was created from parts of Greenwich, and Pohatcong Township went off on its own as of January 1, 1882.