I believe that Potosi has always been in Washington County. But the land where Potosi sits today was once a part of St Francois County, but that was long before Potosi was built. Washington County was created from St Francois County around 1800.

Typically, early American counties were much larger than the counties that we're familiar with today. After the settlers moved in and the population rose, these huge "mega-counties" like St Francois were carved up and subdivided into the smaller counties we have today. Another good example of this was St Louis County: More than 200 years ago it reached from St Louis City to the Kansas state line and included most of the Ozarks. St Louis County was practically all of the land south of the Missouri River to the Arkansas border.