[5]: 1  Archaeological investigation at Skamokawa indicated that the site, where the inhabitants engaged in fishing, hunting, wood-working, and tool manufacture, was approximately 2,300 years old.

[5]: 15 Merchant sea captain Robert Gray, on his Columbia River expedition in 1792, was the first American known to arrive in the area.

During salmon season in the early 1800s, the Hudson's Bay Company's post at Fort George operated a salting station in the region.

The influence of his wife, Charlotte Beaulieu Birnie, whose parents were a French voyageur and a Kootenay tribe member, helped protect Cathlamet during the Puget Sound War.

[5]: 126 Early settlers came from Great Britain and the eastern United States until around 1870, when they were followed by a large number of immigrants from Sweden, Norway, Finland, and Dalmatia.

From 1870 to the early 1900s, there was a large population of Chinese residents who worked in the fish canneries.

[5]: 98  Numerous areas of population disappeared in the years after roads began to replace water transportation.

Salmon canning over the years was impacted by overfishing and habitat degradation, with declines observed by 1889.

[5]: 75–82 As a densely wooded region, Wahkiakum County was an abundant source for logging companies.

When timber nearest the water had been felled, loggers had to rely on oxen, and then on railroads.

[5]: 59  The first chapter in Wahkiakum County of the National Grange of the Order of Patrons of Husbandry was organized in 1901.

[5]: 67 The earliest periodicals in the area included the Cathlamet Gazette (1889), the Skamokawa Eagle (1891) which continues to be the county's official newspaper, and the Timberman (1899).

18.7% were of German, 13.1% Norwegian, 10.6% United States or American, 9.1% English, 7.7% Irish, 6.5% Swedish, and 5.9% Finnish ancestry.

In the 2016 Presidential election, Donald Trump won the county over Hillary Clinton by a decisive margin – 55.3% to 34.3%.