Montesano is a city in, and the county seat of, Grays Harbor County, Washington, United States. The population was 4,138 at the 2020 Ce…Montesano is a city in, and the county seat of, Grays Harbor County, Washington, United States. The population was 4,138 at the 2020 Census. According to Edwin Van Syckle, a portion of the present-day town of Montesano was first platted in 1870 by Samuel Henry Williams, who purchased land in what was known as Medcalf Prairie, named after the early settler William Medcalf. At about the same time, surveyor Charles Newton Byles bought a farm from Walter King on the present-day site of Montesano on April 9, 1870, and later platted three blocks on the west side of Main Street. The town of Montesano was born, but it was not incorporated until November 26, 1883, by the Washington Territorial Legislature.