Montesano’s roots go back to the first settler, Isaiah Scammon and his wife Lorindawho arrived in 1852. He filed a claim on 640 acres along the banks of the Chehalis River. In 1860 his residence was selected as the county seat and also became the first post office. In 1883 Montesano officially became a city with a population of 300. 

Grays Harbor County CourtHouse in Montesano

As the seat of Grays Harbor County, it is one of the oldest cities in the region. Montesano lies at the confluence of the sluggish Wynooche (Ind. “shifting sands”) and Chehalis rivers. The first settlement was established on the south bank of the Chehalis, almost opposite the mouth of the Wynooche, by Isaiah Scammon, who had come from Maine. His journey to this part of Washington is illustrative of the difficulties that beset travelers in his day. He came around the Horn to San Francisco then North to Astoria by boat; crossed the Columbia River to Ilwaco near Fort Canby, then went by stage to what was then known as "Peterson’s Point", now known as Westport. Crossing Grays Harbor he went upstream to the present site of Montesano. The settlement was known for a long time as Scammon’s; later, as Wynooche.

The name Montesano was used to refer to the homestead of Isaiah Scammon and his wife Lorinda. They filed a 640-acre Donation Land Claim on January 15, 1853, on the Chehalis River, opposite the mouth of the Wynoochee River, in the area now referred to as South Montesano. The Scammon home was often referred to as "Scammon's Landing" or "Scammon's Hotel", because it was an important stopping point along the Chehalis River for early pioneers, and the furthest up-river mooring point and railroad junction for seagoing ships.[6]

According to Edmond S. Meany, in 1860 the seat of what was then called Chehalis County was moved to "the place of J.L. Scammons". Mrs. Lorinda Scammon was very religious and wished to have the place named "Mount Zion". Another early settler, Samuel James, suggested "Montesano", which was derived from Spanish meaning "mountain of health"; it was said that this had a more pleasant sound and meant about the same. The suggestion was approved, and soon after, a post office was secured with the name "Montesano". The people of Chehalis County voted to move the county seat to the platted town of Montesano in 1886, and the Scammon home was then known as South Montesano.

Information sourced from Wikipedia