 Of the 172 counties that have ever existed in Virginia's history, Northumberland ended up being an "ancestor" to 116 of these –– more than the current 95 counties (several were lost to other states, such as West Virginia).According to the U.S. Census Bureau, the county has a total area of , of which is land and (33.0%) is water.The county is located between the Rappahannock River to the south and Potomac River to the north.