The county's largest employer is the U.S. Naval Surface Warfare Center Dahlgren Division. It is adjacent to the two-lane, 2-mile long Harry W. Nice Memorial Bridge carrying U.S. Highway 301 over the Potomac River. It contains the ZIP codes 22448 (Dahlgren) and 22485 (all other locations). It is within the area code 540 and contains the exchanges: 775, 644, 663, and 653.
Indigenous peoples of varying cultures lived along the waterways for thousands of years before Europeans arrived. Among the historic Native American tribes who came into conflict with the English were the Algonquian-speaking Nanzatico. In 1704 colonists retaliated for the tribe's attacking the farm of John Rowley, "known for his disputes" with them. They captured and shipped 40 people, including children older than 12, to Antigua in the Caribbean, where they were sold into slavery.
King George County was established in 1720 when land was split from Richmond County, Virginia. The county is named for King George I of Great Britain. It was substantially reorganized in 1776 and 1777, with land swapped with both Stafford and Westmoreland Counties to form the modern boundaries.
On March 16, 1751, James Madison, the fourth President of the United States, was born in Port Conway in southern King George County at Belle Grove plantation, the childhood home of his mother, Eleanor Rose "Nellie" Conway, the daughter of its owner, Francis Conway, for whom Port Conway was named. Two-time Governor of Virginia, William "Extra Billy" Smith, was born at Marengo in 1797.
On May 1, 1861 during the American Civil War, Confederates installed artillery at Mathias Point in order to blockade the Potomac River. On June 27, the steamer Thomas Freeborn bombarded Mathias Point in an attempt to drive away the soldiers who were manning the weapons. Confederate soldiers fired back from Mathias Point, striking and mortally wounding Commander James H. Ward of the Freeborn, who became the first Union naval officer to die in the Civil War.