In March 1849, Lawrence County, Pennsylvania, was established from portions of Beaver and Mercer Counties and originally consisted of thirteen townships. A few years later, in April 1854, the former North Slippery Rock Township—once part of Mercer County—was divided to create Washington and Scott Townships. Then, in February 1855, Plain Grove Township (sometimes spelled Plaingrove) was formed, occupying the northeast corner of Lawrence County by combining sections of both Washington and Scott Townships.