Carroll County is a county located in the U.S. state of Maryland. As of the 2020 United States census, the population was 172,891. Its county seat is Westminster. The county is part of the Central Maryland region of the state. Carroll County is included in the Washington-Baltimore-Arlington, DC-MD-VA-WV-PA Combined Statistical Area. While predominantly rural, southern areas of the county have become increasingly suburban in recent years.

Prior to European colonization, the land that now makes up Carroll County was inhabited by Native Americans for thousands of years. Numerous Native American archaeological sites and archeological artifacts have been located across the county. Native Americans used the land for permanent settlements, seasonal visits and journeys, and as hunting grounds.

At the time of European colonization, the Susquehannock and the Lenape were the predominant indigenous nations in the area. They spoke Iroquoian and Algonquian languages, respectively. Present-day Manchester, which was inhabited by the Susquehannock nation until around 1750, was where the two important Native American trails intersected. An ancient trail that was used by Algonquian and Iroquois nations, named the "Patapsco-Conewago (Hanover) Road" by colonists, stretched from the Susquehanna River to the Potomac River. Main Street in Westminster was built on a portion of the trail between the two rivers.

By the end of the 18th century, most roads in Carroll County followed trails that had been established by Native Americans.

Maryland Route 26 (Liberty Road) was built over what was originally a Native American trail, which passed through the Freedom area of southern Carroll County and was used by Native Americans to travel from the Blue Ridge Mountains to Chesapeake Bay. The trail was developed as a road and renamed "Liberty" by an act of the Maryland General Assembly in the early 1800s. The land of what is now Sykesville was used by the Susquehannock and the Lenape as hunting grounds.

Taneytown was inhabited by the Tuscarora people during the early to mid-1700s. The Tuscarora, also Iroquoian speakers, hunted deer, wolves, wildcats, and otters in the woodlands of what is now Taneytown.

Due to the Six Nations land cessions, the Tuscarora were expelled westward by European Americans across the South Mountain of the Cumberland Valley.

Carroll County was created in 1837 from parts of Baltimore and Frederick Counties. The county was named for Charles Carroll of Carrollton, who was one of 56 delegates to the Second Continental Congress in Philadelphia who unanimously signed the United States Declaration of Independence.