Polk County was officially created from Yamhill District of the Oregon Territory on December 22, 1845. On August 13, 1848, President James K. Polk signed a bill approving the boundaries of the Oregon territory, which officially separated the territory from England. Thus came the name Polk County.

The present area of Polk County is 472,960 acres. Hudson's Bay Company hunters and trappers had penetrated the Willamette Valley as far south as Polk County as early as 1830. Initial settlement of the Willamette Valley started with the establishment of Etienne Lucier's farm at the extreme northwest corner of French Prairie in 1829. French Prairie was colonized thereafter, during the 1830's and 1840's, by retired servants of the Hudson's Bay Company.

White people from the eastern United States began settlement of Polk County during the early 1840's. Jason Lee was actually the vanguard of this settlement, having established his mission at Wheatland on the east bank of the Willamette river in 1834. The original Polk County Courthouse was a wooden two-story building located on the north side of Dallas in a community first named Cynthia Ann, named after a daughter of Thomas J. Lovelady. It was located near the intersection of Ellendale and Main in north Dallas and was completed in 1851.

As a village on the south side of the LaCreole River, now known as the Rickreall Creek, began to grow in size, Cynthia Ann lost its inhabitants to the new community and a new county courthouse was needed. In 1856, John E. Lyle, Isaac Levens, Solomon Shelton and others sought a charter for LaCreole Academy. They funded the project by selling lots in the original town of Dallas as the plat stated. One block was set aside as a courthouse square. This second courthouse was built in 1859. It was a two-story wood structure of classical design. It was said to be modeled after the state capitol in Richmond, Virginia. After years of controversy over the site and its inadequacies, the building burned to the ground the night of June 10, 1889. Arson was strongly suspected and the county court offered a reward for information, but the reward was never collected.