Seymour was founded by settlers from Oregon, who called the town Oregon City; it was originally located where the Western Trail crossed the Brazos River, which flows just south of the townsite. A post office was established in 1879, when the town's name was changed to honor local cowboy Seymour Munday, after whom nearby Munday was also named. The census of 1880 shows 78 people living in the 901-square-mile county. The first settlers were ranchers. During those years, the Millett brothers established a large ranch with the headquarters some 10 miles south of Seymour on Miller Creek. About 50 men were in the Millett outfit, and some of these were reportedly outlaws. Not unusually, some of the cowboys rode into town and proceeded to "shoot it up". The Milletts realized that law had come to Baylor County in 1884, and sold the ranch. It was the largest transaction ever made in northwest Texas at that time. The purchasers used the Hashknife Ranch brand, which is still in use in the county.