There are 4 US Post Offices in Rockcastle County serving a total of 14,140 residents equating to 3,535 residents per post office. It's estimated that approximately 22,569 packages are handled across the 4 county post offices annually. Rockcastle County is a county located in the U.S. state of Kentucky. As of the 2010 census, the population was 17,056. Its county seat is Mt. Vernon. The county founded in 1810 and named for the Rockcastle River which runs through it. The river, in turn, is named for its majestic rock cliffs. Rockcastle County is part of the Richmond\u2013Berea, KY Micropolitan Statistical Area, which is also included in the Lexington-Fayette-Richmond-Frankfort, KY Combined Statistical Area."