[3] Its name derives from the anglicized version of the Cowlitz Indian term Cow-e-liske, meaning either 'river of shifting sands' or 'capturing the medicine spirit.

The Cowlitz are considered to be the first regional inhabitants to engage in commerce as they traded extensively with other tribes in Western and Eastern Washington.

By the 1820s, the Hudson's Bay Company had established a lucrative fur trade in the region.

Trade declined significantly in the late 1830s as over-hunting reduced the annual yields, and wearing fur had become less fashionable.

Most of the settlers homesteaded near the tributaries that fed the Columbia River, forming settlements.

In 1841 several families with the HBC directed Sinclair expedition from Red River Colony settled there.

On November 25, 1852, at Monticello, settlers from the Cowlitz and Puget Sound regions drafted a petition (the Monticello Convention) to the federal government, calling for a separate territory north of the Columbia River to be carved out of the existing Oregon Territory.

In December of that same year, the Oregon Territorial government sliced off the eastern portion to create Lewis County.

This proclamation was finalized on April 24, 1854, signed into law by Governor Isaac Stevens.

Nearly every town that sprang up in the late 19th century began around a logging or lumber-milling operation.

Nestled against the Cascade Mountains, many of the county's major rivers originate in this range, including the Columbia, Cowlitz, Coweeman, Kalama, Lewis and Toutle.

Donald Trump won a majority of the vote in 2016, 2020, and 2024, becoming the first Republican to win this county since Reagan in 1980.