Johnston County is a county located in the U.S. state of North Carolina. As of the 2020 census, the population was 215,999. Its county seat is Smithfield. Johnston County is included in the Raleigh-Cary, NC Metropolitan Statistical Area, which is also included in the Raleigh-Durham-Cary, NC Combined Statistical Area, which had an estimated population of 2,368,947 in 2023.