Attracted by the waterways and longleaf pine forests, the first European and English settlers arrived here in 1713 in what was originally part of the colonial precincts of Carteret and New Hanover. Onslow County was formed in 1734 and was named for the Honorable Arthur Onslow, Speaker of the British House of Commons. After a lethal 1752 hurricane, the county courthouse was relocated from Town Point to Wantland's Ferry; this settlement was eventually incorporated in 1842 and named Jacksonville after President Andrew Jackson. Largely a collection of sparsely populated agrarian and maritime communities, Onslow County dramatically changed in the early 1940s with the establishment of the Army's Camp Davis near Holly Ridge (now closed), and the creation of Camp Lejeune in 1941.

Onslow County's flat, gently rolling terrain covers 767 square miles and is located in the southeastern coastal plain of North Carolina, approximately 120 miles east of Raleigh, and 50 miles north of Wilmington. The city of Jacksonville is the county seat, and the areas surrounding the city constitute the major population centers and growth areas in the county. The county is home to more than 150,000 people and includes the incorporated towns of Holly Ridge, Richlands, Swansboro, North Topsail Beach, part of Surf City, and unincorporated Sneads Ferry. Approximately 156,000 acres comprise the U.S. Marine Corps Base, Camp Lejeune and more than 43,000 marines and sailors are stationed there.

The structure of local government in Onslow County was established in the late 19th century and consists of a five-member Board of Commissioners, elected at large for four-year terms. The Board establishes policies and ordinances implemented by the County Manager and his staff.