[2] The area delineated by the future Washington state boundary began to be colonized at the start of the nineteenth century, both by Americans and British.

The Treaty of 1818 provided for the creation of a British and American condominium over the region.

However, the condominium arrangement was unwieldy, leading to continuous disputes and occasional conflict; it was abolished by an 1846 treaty that established a boundary between British and American possessions that survives as today's Canada–United States border.

[3][4] It was named for the late U.S. President James A. Garfield,[5] who had been assassinated a few weeks prior.

[6] It is part of the Palouse, a wide and rolling prairie-like region of the middle Columbia basin.

[12] In terms of ancestry, 27.2% were German, 22.4% were English, 19.9% were Irish, 7.9% were Dutch, 5.5% were Swedish, and 3.4% were American.

28.8% were of German, 17.9% United States or American, 10.6% English and 9.5% Irish ancestry.

[16] Garfield County has consistently voted for Republican candidates in presidential elections throughout its history.

In 1912 the plurality of votes went instead to the Progressive Party candidate, former Republican president Theodore Roosevelt.