Onslow County is a county located in the U.S. state of North Carolina. As of the 2020 census, the population was 204,576. Its county seat is Jacksonville. The county was created in 1734 as Onslow Precinct and gained county status in 1739. Onslow County comprises the Jacksonville, NC Metropolitan Statistical Area. The southern border is the coast of the Atlantic Ocean.

Jacksonville is a city in Onslow County, North Carolina, United States. As of the 2020 census, the population was 72,723, which makes Jacksonville the 14th-most populous city in North Carolina. Jacksonville is the county seat and most populous community of Onslow County, which is coterminous with the Jacksonville, NC Metropolitan Statistical Area. Demographically, Jacksonville is the youngest city in the United States, with an average age of 22.8 years old, which can be attributed to the large military presence. The low age may also be in part due to the population drastically increasing over the past 80 years, from 783 in the 1930 census to 72,723 in the 2020 census.