Andrew Ellicott (January 24, 1754 – August 28, 1820) was an American land surveyor who helped map many of the territories west of the Appalachians, surveyed the boundaries of the District of Columbia, continued and completed Pierre (Peter) Charles L'Enfant's work on the plan for Washington, D.C., and served as a teacher in survey methods for Meriwether Lewis.

Andrew Ellicott was born in Buckingham Township, Bucks County, Pennsylvania as the first of nine children of Joseph Ellicott (1732–1780) and his wife Judith (née Blaker or Bleaker, 1729–1809).^{[unreliable source?]} The Quaker family lived in modest conditions; his father was a miller and clockmaker. Young Andrew was educated at the local Quaker school, where Robert Patterson, who later became a professor and vice provost at the University of Pennsylvania in Philadelphia, was his teacher for some time. Andrew was a talented mechanic like many of the family and showed some mathematical talent, too.

In 1770, his father, together with his uncles Andrew and John, purchased land on the falls of the Patapsco River, upriver and west of Baltimore and the Chesapeake Bay. There they set up a new milling business, founding the town of Ellicott's Mills in 1772 (today's Ellicott City, Maryland). Three years later, Andrew married Sarah Brown (1756/8–1827) of Newtown, Pennsylvania, with whom he would have ten children, one of which died as a child. When the Revolutionary War broke out in 1775, Andrew enlisted as a commissioned officer in the Elk Ridge Battalion of the newly organized Maryland state militia despite his Quaker upbringing. During the course of the war, he rose to the rank of major, a title he would keep as an honorific throughout his life.

After the war, Ellicott returned home to Ellicott's Mills until he was appointed, in 1784, a member of the survey group tasked with extending the survey of the Mason-Dixon line for the borders between Pennsylvania / Delaware with Maryland that had been abandoned in 1767 and then been stalled during the war. In this survey, he worked alongside David Rittenhouse and Bishop James Madison, making first connections with the scientific society of Philadelphia.

Following the death of their second son, the Ellicotts moved to Baltimore in 1785, where Andrew taught mathematics at the Baltimore Academy and was even elected to the Maryland General Assembly (state legislature) in 1786. The same year, he was called upon for a survey to define the western border of Pennsylvania with the Ohio Country. This "Ellicott Line" (running north–south at longitude meridian 80°31′12″W) later became the principal meridian for the surveys of the future Northwest Territory of the United States. His work in Pennsylvania intensified his ties with Rittenhouse and other members of the American Philosophical Society, to which he had been elected a member in 1785, and led to encounters with Benjamin Franklin and Simeon De Witt. When he was subsequently appointed to lead other surveys in Pennsylvania, the family moved again in 1789 to Philadelphia. By recommendation of Franklin, Ellicott got a position with the newly established government under the Constitution and was tasked by first President George Washington to survey the lands between Lake Erie and Pennsylvania to determine the border between Western New York and U.S. federal territory, resulting in the Erie Triangle. This survey, during which he also made the first topographical study of the Niagara River including the Niagara Falls, gained Ellicott a reputation for superb accuracy in surveys.

From 1791 to 1792, at the request of Secretary of State Thomas Jefferson, Ellicott worked under the direction of the three commissioners that President George Washington had appointed, surveying the boundaries of the federal Territory of Columbia, which would become the District of Columbia in 1801, containing the Federal City also then becoming known as "Washington City". Ellicott began his survey in February 1791, just prior to the arrival of L'Enfant. He was assisted in this survey first by the free African-American astronomer Benjamin Banneker and then by Ellicott's brothers, Joseph Ellicott and Benjamin Ellicott. Ellicott's team put into place forty boundary stones approximately 1 mile (2 km) apart from each other that marked the borders of the Territory of Columbia of 100 square miles (260 km²) (see: Boundary Markers of the Original District of Columbia). Most of these stones remain in their original positions. As engravings on many of the stones still show, Ellicott's team placed those that marked the southwestern /southeastern border with Virginia in 1791, and those that marked the northwestern / northeastern border with Maryland in 1792.

On January 1, 1793, Ellicott submitted to the three commissioners "a report of his first map of the four lines of experiment, showing a half mile on each side, including the district of territory, with a survey of the different waters within the territory". The Library of Congress has attributed to 1793 Ellicott's earliest map of the Territory of Columbia that the Library holds within its collections.

During 1791–1792, Ellicott also surveyed the future city of Washington, which was located within a relatively small area at the center of the Territory of Columbia along the northern bank of the Potomac River at the confluence with its Eastern Branch (known today as the Anacostia River). Ellicott also served under the Commissioners' supervision in this effort. He first worked with Pierre (Peter) Charles L'Enfant, who had prepared the initial plans for the future capital city during the early months of 1791 and had presented one of these early plans to President Washington in August of that year (see L'Enfant Plan).