Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Washington County is a county established in 1849 in the U.S. state of Minnesota. As of 2000, the population was 201,130. Its county seat is Stillwater.ne of the nine original counties created when the Minnesota Territory was organized in 1849. The county was officially established October 27, 1849, named after George Washington. Early development in the area was on the St. Croix River, which forms the eastern boundary of the county. The river not only provided a means of transportation to move people upstream, but also move logs downstream. The area was heavily forested and the early economy was dependent on the logging and lumber industries.