St. Croix County was created in 1840 by the Wisconsin Territorial legislature from….
The original St. Croix County seat was Stillwater on the western bank of the St. Croix River. However, when Wisconsin became a state in 1848, the western boundary of the State of Wisconsin was fixed at the St. Croix River, leaving Stillwater in what eventually became the State of Minnesota (Stillwater is the county seat of Washington County in Minnesota). The revised St. Croix County had its county seat at St. Croix Falls. In 1853, the state legislature split St. Croix County into three parts. The northern part became Polk County and the southern part became Pierce County. The central part remained St. Croix County but it again needed a new county seat because St. Croix Falls was now in Polk County. Hudson was chosen as the new county seat.