The earliest known documentary record of the county is dated January 12, 1659, when a writ was issued on behalf of the General Assembly of Maryland to its sheriff.

Prior to 1674, Baltimore County court sessions were held in private residences, according to sketchy documentary evidence.

The exact location of Old Baltimore is Chilbury Point on the north side of the Bush River owned by the Garrison of the present-day Aberdeen Proving Ground (APG), a U.S. Army weapons testing facility.

Goodwin first performed historical and archival work and coordinated with existing landscape features to locate the site of Old Baltimore.

Goodwin dug 420 test pits, uncovering artifacts including a King Charles II farthing coin, and French and English gun flints.

This change of location, away from the Bush River area, reflects the growing economic and political importance of the Gunpowder region.

The assembly's ordinance directed that the land be divided into 40 lots with streets and alleys to accompany the courthouse and jail erected previously.

By 1750, about 50 houses (including a few large two-story brick structures), a church (St. John's Anglican Parish), a courthouse, three stone warehouses, inns, taverns, stores, a public wharf and a "gallows-tree" with an "Amen Corner" with pillories and whipping posts (now located northeast of the City of Baltimore near present-day suburban "Joppatowne" off Harford Road) existed.

A new port and wharfing site, Elkridge Landing, on the upper Patapsco River's Western Branch, became prosperous in the 18th century.

The landing was a designated "port of entry" and was the terminus of several "rolling roads" on which horse or oxen-drawn hogsheads (huge barrels) packed with tobacco were wheeled down to the Landing/port to be loaded on ships sailing for London and Europe.

Gradually the site silted-up from soil erosion and poor farming cultivation on the upper Patapsco, and the maritime economy of the Landing faded.

In the 19th century, it became an important stop on the Baltimore and Ohio Railroad and the main north-south East Coast highway for wagons and carriages.

In 1768, following receipt of petitions for and against the relocation, the General Assembly passed an Act that moved the county seat from Joppa to Baltimore.

[5] The site of the courthouse is now "Battle Monument Square", constructed 1815–1822 to commemorate the city and county defense in the War of 1812, including the bombardment of Fort McHenry by the British Royal Navy fleet in the Patapsco River, the two-day stand-off in fortifications dug east of the city on Loudenschlager's Hill (now "Hampstead Hill" in today's Patterson Park) and the earlier Battle of North Point in "Godly Woods" on the "Patapsco Neck" peninsula in the southeastern portion of the county, during September 12–14, 1814.

These events have been commemorated ever since by Defenders Day, an annual city, county, and state official holiday on September 12.

A second city-county courthouse constructed in 1805–1809 was moved to the western side of the Square at North Calvert and East Lexington.

The County separated from the city (which it surrounds on the east, north, and west) on July 4, 1851, as a result of the adoption of the 1851 second state constitution.

A new Baltimore County Courthouse was authorized to be built facing Washington Avenue, between Chesapeake and Pennsylvania Avenues to replace the previous courthouse and governmental offices then centered for near 85 years in the city, which had been the official "county seat" since just before the American Revolution.

Later surrounded by manicured flower gardens, shrubs and curved walkways, the historical landmark is built of local limestone and marble.

Extensive city-county hostilities came during the Civil Rights Movement, and by the 1980s the county's older inner suburbs faced increasing urban social ills.

Without incorporated cities or towns, the county government provides all local services to its residents, many of which are normally associated with city-type governmental agencies.

He was later accused of corruption and bribery while serving as County executive and continuing to accept bribes as the state's governor and as U.S. vice president.

While his primary duty was to serve the Proprietor, the sheriff was aware of problems faced by poor planters and tradesmen.

With taxes, yearly quit-rents and other costly expenditures, many of the poorer settlers were unable to pay their obligations when due.

Along with neighboring Howard County, it has voted for the state-wide presidential winner in 10 straight elections, the longest such streak in the state.

[25] The highest elevation is approximately 960 feet (290 m) above sea level at Maryland's state border with Pennsylvania near Steltz.

Commuter MARC Train service is available in the county at Halethorpe, St. Denis, and Martin State Airport stations.

The present-day streetcar/trolley line coming north from Anne Arundel County and the International Airport through Baltimore City uses the Northern Central right-of-way south of Cockeysville and Timonium; starting slightly north of that, the right-of-way was converted into the popular hiking, biking and jogging pathway from Loch Raven to the Mason–Dixon line with Pennsylvania known now as the Torrey C. Brown Rail Trail, named for a former state secretary of natural resources.

[42] Among the county's major employers are MedStar Franklin Square Medical Center[43] on the east side in Rossville, the Social Security Administration, the national headquarters of which are in Woodlawn, and The Black & Decker Corporation, in Towson.

[44] As of 2009, the county's workforce totaled 410,100, with 25% employed in the fields of education, health and human services, 10% in retailing, and less than 1% in agriculture.