These areas comprised part of Westmoreland until the formation of Stafford County in 1664.

Westmoreland County on Northern Neck was the birthplace of George Washington, who later became the first President of the United States (born at the former settlement of Bridges Creek, Virginia);[4] of James Monroe, the fifth President; and of Robert E. Lee, general and commander of the Confederate armies during the American Civil War of 1861–1865.

Spencer, who served as President of the Council and acting Governor (in office: 1683–1684) of the Colony of Virginia, was the cousin of, and agent for, the Barons Colepeper, proprietors of the Northern Neck.

Spencer lived at his plantation, Nomini, which his descendants later sold to Robert Carter I (1662/63 – 1732) .

[citation needed] This manumission was the largest known release of slaves in North America prior to the American Civil War and involved the largest number ever manumitted by an individual in the U.S.[5] According to the U.S. Census Bureau, the county has a total area of 253 square miles (660 km2), of which 229 square miles (590 km2) is land and 24 square miles (62 km2) (9.3%) is water.

Tourism is another significant economic driver, related to historical sites such as George Washington Birthplace National Monument and Robert E. Lee's birthplace, Stratford Hall Plantation, and the Westmoreland County Museum as well as gambling activities available in Colonial Beach.

The county is also an extended exurb of Washington, D.C. Northern Neck Coca-Cola Bottling Inc. (makers of Northern Neck Ginger Ale) and the weekly Westmoreland News are located in Montross.

Westmoreland County is a notable bellwether for U.S. presidential politics, having voted for the winner in every election since 1928 except 1948, 1960, and 2020.