It was once an important sawmill town, but when the largest sawmill closed, the town became a quiet, residential community. It has been revived by the building of a modern sawmill and a wood pulp plant. The name was given in pioneer days by a Frenchman named Brunn. Local legend says the name came from an Indian headman who lived nearby. August V. Kautz visited the place in the 1850s on a military expedition and found it to be empty of people.
By the 1940s the town consisted of a collection of old frame dwellings, many of them vacant, showing the harsh effects of the decline of lumbering in the region. The inhabitants of Cosmopolis were mostly millworkers of many nationalities. The town was one of the first sawmill centers established in the Grays Harbor district.
Weyerhauser’s Cosmopolis Pulp Mill is presently the dominant feature of the landscape and it has brought a measure of vitality to the town. Many of the frame buildings demonstrate a level of sensitivity in their treatment identifying them as early twentieth century resources.