William P. Overton was in Oregon fewer than three years, yet he made a mark on Oregon history. In May 1841, Overton and others from Jackson County, Missouri, joined the John Bidwell and John Bartleson party, which intended to journey to California. Overton was one of at least twenty-three people who left the party at Soda Springs and traveled to Oregon with Father Pierre De Smet and other Jesuit missionaries.

Overton arrived at the Methodist Mission at The Dalles on September 24. He worked and boarded there from November 1 until January 5, 1842, when he left for Fort Vancouver. He met Asa Lovejoy there in November 1843.

The two men set out in a canoe to go to Oregon City. On the way, they stopped on the shore of the Willamette River near a grove of trees known as "the clearing," which Indians had cleared of underbrush. It would be a good location for a townsite, they decided, and filed a claim for what would become the site of downtown Portland.

Lovejoy filed a Provisional Government land claim on 640 acres because Overton reportedly did not have the 25¢ filing fee, and the two became partners. Overton stayed there during the winter, building a crude shelter and making shingles and staves that he sold in Oregon City. In June 1844, a destructive flood on the Willamette discouraged Overton, and he sold his half-interest in the claim to Francis W. Pettygrove for $50 in goods and supplies.

Overton left Oregon in the summer of 1844. Rumors persisted about his whereabouts, and in 1875 James W. Nesmith reported to the Oregon Pioneer Association that he had heard that Overton was hanged in Texas. There is evidence, however, that he relocated to Hawaii after leaving Oregon.