Johnston County is a county located in the U.S. state of North Carolina. As of the 2010 census, the population was 168,878. Its county seat is Smithfield. Johnston County is included in the Raleigh, NC Metropolitan Statistical Area, which is also included in the Raleigh-Durham-Chapel Hill, NC Combined Statistical Area, which has a population of 1,998,808 as of U.S. Census 2012 Population Estimates.