The county seat is Jacksonville, the largest city with roughly 73,000 residents. Jacksonville serves as the main hub and is heavily tied to Marine Corps Base Camp Lejeune, which lies mostly within the county.
Other key communities include:

• Swansboro – about 3,700 residents, a charming historic coastal town on the White Oak River.
• Richlands – around 2,000 people, a small inland town.
• Holly Ridge – about 4,500 residents, a rapidly growing area near the base and beaches.
• Surf City – roughly 3,000 people (partly in Pender County), a popular beach town on Topsail Island.
• North Topsail Beach – small beach community with about 1,000 residents.

The total population of Onslow County is approximately 200,000, making it one of the larger coastal counties. A significant portion of the population is connected to the military at Camp Lejeune, with growth concentrated around Jacksonville and the beach communities, while much of the interior remains rural with pine forests and farmland.