The area that comprises modern-day Grays Harbor County is the ancestral territory of several indigenous Coast Salish peoples, including the Quinault and Lower Chehalis.

[3] The first permanent white resident in the future county was William O'Leary, an Irish immigrant who settled on the south side of Grays Harbor in 1848.

The American Provisional Government of Oregon, which had been established prior to the treaty, created Lewis County on December 21, 1845.

[6][7] The first county seat was at Bruceport on Willapa Bay until a referendum in 1860 moved it to the homestead of the Scammon family in modern-day Montesano.

[3] The settlements of Aberdeen and Hoquiam were platted along the north side of Grays Harbor and incorporated by 1890.

A second plan to create a new county, named Grays Harbor, was approved by the Washington State Legislature but ruled unconstitutional by the Supreme Court on February 27, 1907.

The name change, which took effect on June 9, eliminated confusion with the town of Chehalis in neighboring Lewis County.

[9] Due to the flat topography of the county and the drainage of the Chehalis River watershed into the Pacific Ocean, the region experiences repeated floods.

In addition to tidal surges and coastal flooding, high waters stemming from excessive rainfall events are often difficult to shed.

Grays Harbor used to be one of the most consistently Democratic counties in the nation, due to its unionized logging industry.

[23] Principal economic activities in Grays Harbor County include wood and paper production, food processing (especially seafood), and manufacturing.

Mostly consisting of pump houses in flood-prone communities, the efforts also include the construction of floodwalls and levees.

The ongoing project is estimated to cost over $50 million as of 2024[update], providing protection to over 5,100 residences and 1,300 businesses.