By Act of November 12, 1738, Frederick County, Virginia, was created from the western lands of Orange County. Extending west and northwest from the Blue Ridge Mountains, Frederick was bound on the north by the Potomac River and on the south by Augusta County. Frederick originally encompassed all the land that was to become the northern half of West Virginia. Among the Virginia counties eligible for inclusion in West Virginia, Frederick was the only one to vote not to join the new state.

The earliest recorded land grants were to John and Isaac Van Meter, who received 40,000 acres each on June 17, 1730. Receiving a concession of land from the Van Meters, Joist Hite led a party of some 16 families into the lower Shenandoah Valley in 1732. Settling a few miles south of Shawnee Springs, Hite's followers established a settlement known as Opequon. In 1743, when the county's government was established, James Wood laid out Frederick Town on the site of Opequon. In 1752, Frederick Town was renamed Winchester. It is the county seat of Frederick County.

Frederick was divided into two counties in 1753. The part west of the ridge of the Allegheny Mountains became Hampshire County, now West Virginia. Named for the English shire, Hampshire was to become the oldest county in the future state of West Virginia.

While in command at Fort Cumberland, George Washington was elected to the House of Burgesses from Frederick County for the 1758—61 and 1761—65 sessions. After the French and Indian War, people began moving west and Frederick County experienced an increase in population. In 1772, Frederick again divided, this time into three counties. The northern end became Berkeley County, now West Virginia, the central portion remained Frederick, and the southern end became Dunmore (renamed Shenandoah in 1778).

Frederick Countians were early in their assertion of American liberties. On June 8, 1774, the freeholders met and adopted the Frederick County Resolves. Declaring it the inherent right of British subjects to be governed and taxed by their elected representatives, these Frederick Countians considered those acts of Parliament affecting internal policies of the colonies an unconstitutional invasion of their rights. Appointing a committee of correspondence, the freeholders deemed it important to communicate with other such committees. When war came, Capt. Daniel Morgan's rifle company left Frederick for service in the Continental Line.

Following the Revolution there were no immediate changes in the boundaries of Frederick, but in 1836 Clarke and a portion of Warren counties were carved from Frederick reducing it to its present size of 426 square miles. By 1860, Frederick had some 13,000 inhabitants. During the Civil War the county was the scene of numerous engagements. In 1863, Frederick County citizens voted not to become part of West Virginia, while neighboring Jefferson and Berkeley voted to join the new state.