For many years, Native Americans had traversed the county on well-worn trails to reach fishing, hunting, foraging, and trading areas. Many of these trails are still visible in the rangeland. The first non-native people in the area were Americans following the Oregon Trail to the Willamette Valley. In the late 19th century, settlers arrived from the midwestern and eastern United States and Europe to stay and build farms and communities. Many settlers were also part of a larger reverse migration of people who had originally settled in the Willamette Valley.

The Legislative Assembly created Gilliam County on February 25, 1885, from the eastern third of Wasco County after residents complained that they were too far from the county seat in The Dalles. The first county seat was at Alkali, now Arlington. The question of a permanent county seat was placed on general election ballots in 1886, 1888, and again in 1890, when voters chose to move the county seat to Condon, known to early settlers as "Summit Springs." Once the question of the location of the county seat was settled, voters in Gilliam County proved reluctant to provide a courthouse in Condon. The county government operated out of a two-room house until 1903, when the county court appropriated money to construct a courthouse.