Prior to the European settlement, the land now known as Montgomery County was covered in a vast swath of forest crossed by the creeks and small streams that feed the Potomac and Patuxent Rivers. A few small villages of the Piscataway, members of the Algonquian people, were scattered across the southern portions of the county. North of the Great Falls of the Potomac, there were few permanent settlements, and the Piscataway shared hunting camps and foot paths with members of rival peoples like the Susquehannocks and the Senecas. Captain John Smith of the English settlement at Jamestown was probably the first European to explore the area, during his travels along the Potomac River and throughout the Chesapeake region.
These lands were claimed by Europeans for the first time when George Calvert, 1st Baron Baltimore was granted the charter for the colony of Maryland by Charles I of England. However, it wasn't until 1688 that the first tract of land in what is now Montgomery County was granted by the Calvert family to an individual colonist, a wealthy and prominent early Marylander named Henry Darnall. He and other early claimants had no intention of settling their families. They were little more than speculators, securing grants from the colonial leadership and then selling their lands in pieces to settlers. Thus, it was not until approximately 1715 that the first English settlers began building farms and plantations in the area.
These earliest settlers were English or Scottish immigrants from other portions of Maryland, German settlers moving down from Pennsylvania, or Quakers who came to settle on land granted to a convert named James Brooke in what is now Brookeville. Most of these early settlers were small farmers, growing a variety of subsistence crops in addition to the region's main cash crop, tobacco. They transported the tobacco they grew to market through the Potomac River port of Georgetown. Sparsely settled, the area's farms and taverns were nonetheless of strategic importance as access to the interior. General Edward Braddock's army traveled through the county on the way to their disastrous defeat at Fort Duquesne during the French and Indian War.
Like other regions of the American colonies, the future Montgomery County saw protests against British taxation in the years before the American Revolution. Following the signing of the Declaration of Independence, representatives of the area helped draft the new state constitution and begin to build a Maryland free of proprietary control. The new state legislature formed Montgomery County from lands that had at one point or another been part of Charles, Prince George's, and Frederick Counties, naming it after General Richard Montgomery. The leaders of the new county chose as their county seat an area adjacent to Hungerford's Tavern near the center of the county, which would later become Rockville. The newly formed Montgomery County supplied arms, food, and forage for the Continental Army during the Revolution, in addition to soldiers. In 1791, portions of Montgomery County, including Georgetown, were ceded to form the new District of Columbia, along with portions of Prince George's County, Maryland, as well as parts of Virginia that were later returned to Virginia.
In 1828, construction on the C&O Canal commenced and was completed in 1850. Throughout the 19th century, agriculture dominated the economy in Montgomery County, with slaves playing a significant role. In the 1850s, crop production shifted away from tobacco and towards corn. Montgomery County was important in the abolitionist movement, with slave Josiah Henson, who wrote about his experiences in a memoir which became the basis for Harriet Beecher Stowe's Uncle Tom's Cabin (1852). Josiah, the inspiration for the character "Uncle Tom", was a slave in the county and a slave cabin where he is believed to have spent time still stands at the end of a driveway off Old Georgetown Road. In the 1860 presidential election, Montgomery County was one of only four Southern counties to vote for Abraham Lincoln.
Until 1860, only private schools existed in Montgomery County. Initially, schools for European American students were built, and in 1872 schools for African-Americans were added.
In 1873, the Metropolitan Branch of the Baltimore & Ohio Railroad opened, with a route between Washington, D.C. and Point of Rocks, Maryland. The railroad spurred development at Takoma Park, Kensington, Garrett Park, and Chevy Chase.
On July 1, 1997, Montgomery County annexed a portion of Prince George's County, after residents of Takoma Park, which spanned both counties, voted to be entirely within the more affluent Montgomery County.
The county has a number of sites on the National Register of Historic Places.