[3] The county borders southern Pennsylvania to the north, Northern Virginia to the south, and the Martinsburg Panhandle of West Virginia to the south and west.

Washington County is included in the Hagerstown-Martinsburg, MD-WV Metropolitan Statistical Area, which is also included in the Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area.

Washington County thus originally included the entire western part of the state.

[4] A number of properties in the county are listed on the National Register of Historic Places.

[6] Washington County is located in the Appalachian Mountains, stretching from the Ridge-and-Valley Country in the west to South Mountain in the east, which is an extension of the Blue Ridge.

The county is bordered to the north by the Mason–Dixon line with Pennsylvania, to the south by the Potomac River and the states of Virginia and West Virginia, to the west by Sideling Hill Creek and Allegany County, Maryland, and to the east by Frederick County and South Mountain.

The racial makeup of the county was 89.71% White or Caucasian, 7.77% Black or African American, 0.18% Native American, 0.80% Asian, 0.04% Pacific Islander, 0.46% from other races, and 1.04% from two or more races.

26.00% of all households were made up of individuals, and 11.10% had someone living alone who was 65 years of age or older.

As of the 2010 United States Census, there were 147,430 people, 55,687 households, and 37,506 families residing in the county.

[13] In terms of ancestry, 31.7% were German, 14.1% were Irish, 9.8% were English, 8.5% were American, and 5.1% were Italian.

Like most of Appalachia, German-influenced and Unionist Western Maryland,[17] Washington County is solidly Republican.

The last Democrat to carry Washington County at a Presidential level was Lyndon Johnson during his 1964 landslide win over Barry Goldwater, although between 1888 and 1940 the county was a consistent bellwether for all Presidential elections.

According to the Maryland Department of Business and Economic Development, the following were the major employers in the county (excluding post offices, state government, and local governments, but including public institutions of higher education):[23] Washington County is top in the state for commercial production of fruits, tree nuts, and berries.