On March 2, 1899, the Washington State Legislature approves the incorporation of Prosser -- at the time located in Yakima County -- as a Fourth Class City. Six years later, Benton County will be carved out of parts of Yakima County and Klickitat County, and Prosser will become -- and remain, despite several attempts by residents of larger Kennewick to claim the prize -- its county seat.
Prosser, located on the Yakima River in the Columbia Basin region of Eastern Washington, was named for Colonel William F. Prosser (1834-1911), one of the community's settlers. Colonel Prosser had fought for the Union during the Civil War and afterward moved to Tennessee, where he was elected to the state House of Representatives and then to a single term in the U.S. House of Representatives.
Prosser moved to Washington Territory in 1879, after being appointed by President Rutherford B Hayes (1822-1893) as a special agent of the Interior Department for Oregon, Washington, and Idaho. Prosser claimed a homestead near the present town site in 1882. One year after the arrival of the Northern Pacific Railroad, the town was platted as Prosser Falls. In 1886, Colonel Prosser was elected Yakima County auditor and moved to North Yakima, never to return to the town he founded.
The community -- which later shortened its name to Prosser -- prospered, aided by the construction of a flour mill in 1887 and the arrival of irrigation water in 1893. Before century's end, the community's population exceeded 300, more than enough to incorporate as a city under state law.
On January 29, 1899, 60 electors from the community of Prosser appeared before the Yakima County Commission and petitioned to hold an election of incorporation. Their request was granted, and the election was held on February 14, 1899. The vote was 40 to 18 in favor of becoming a city. The Washington State Legislature formally approved Prosser's incorporation on March 2, 1899.
In 1905, the legislature created Benton County out of parts of Yakima and Klickitat County. In the process, Prosser was named county seat. This did not sit well with citizens in Kennewick and Benton City, who felt that their own towns should be the seat. An election was held in 1912 to decide the issue, but although Kennewick -- with a larger population -- received more votes, it did not reach the 60 percent threshold required by law to move the seat.
Kennewick residents tried to wrest control of the county seat several times over the years since, but to no avail -- in 2013, the seat remains in Prosser (although courts, the jail, and many other county offices are located in Kennewick).