History:
In February, 1855, Governor Isaac Stevens called a conference with the area Indians on the banks of the Chehalis River in what is now the City of Cosmopolis. He had recently signed with the Makah Indians and wanted agreements with all the Indian tribes. James J. Swan in the book "Three Years Residence in Washington Territory" (1857 Harper & Bros. New York) said there were 350 men, women, and children in Indian dress "brightest tribal colors and very picturesque". Each tribe had a space allotted to it. The Coast Indians were placed at the lower part of the camp: First the Chinooks, then the Chehalis, Quen-ai-ult, and Quaitso, Satshop, upper Chehalis, and Cowlitz...the best of feeling prevailed among all. The conference was inconclusive, but Governor Stevens, on January 25, 1856, signed a formal treaty document with the Quinault and Quileute tribes.

James Pilkington sold the land to David F. Byles and Austin E. Young on April 26, 1859 for $1500. On June 25, 1861, the streets and alleys were dedicated to the public on consideration that a town called Cosmopolis be laid out.

In 1860 a brickyard was opened and in 1861, a tannery. A grist mill opened in 1879 to grind wheat, but was not a success because it was too expensive to haul grain by canoe. In 1888 the Grays Harbor Mill Company opened the first saw mill in the world on the banks of the Chehalis. In 1957, the Weyerhaeuser Company purchased the property for a pulp mill, producing grades of pulp in such diverse products as photographic papers, plastic molding compounds, acetate yarns, cigarette filters, toothpaste, and air-laid toweling.

Cosmopolis is the oldest city on Grays Harbor, since a donation land claim filed there in 1852. It was incorporated in 1891, and is a code city. According to James Phillips, Washington State Place Names, the city was named in 1853 and means "city of the world". Local people prefer, with noticeable affection, to use the city's nickname, "Cosi".

In the 1960s, 1970s, and in the late 1990s and early years of the new century, Cosmopolis developed residential areas on the hills. Although the population has not risen considerably, the area continues to enjoy a larger median income population than other areas in Grays Harbor and a distinct "pride" of the community. Cosmopolis has three larger parks: Lions Club boasts a community field and play equipment; Mill Creek Park boasts a juvenile fishing pond, trails, and tennis courts; Makarenko Memorial Park has two miles of trails in a natural setting, soccer fields, and a community garden. In 1997, a new fire station was constructed and in 2001 beautification enhancements occurred on 1st and 2nd Streets.