Born on January 17, 1726, in Roseharty, Scotland, Hugh Mercer got an early start on his military career. The son of Reverend William Mercer and his wife Ann, Mercer attended Marischal College at the University of Aberdeen. When he graduated at the age of twenty, he became a Doctor. He quickly jumped into the military as an assistant surgeon in the Jacobite Army under Prince Charles Edward Stuart. The Jacobites attempted to oust the House of Hanover from the British throne and to restore the Scottish House of Stuart to that position. He ended his career there when the Prince was defeated by the Duke of Cumberland at the Battle of Culloden in 1746.

Mercer was forced to leave his home country after the defeat of the Prince in 1747, landing in Philadelphia. He started up his medical career for eight years before going back to the military. Mercer found himself fighting under the British flag in the French and Indian War, the American theatre of what was known in Europe as the Seven Years War. Following the disastrous defeat of the British forces during General Braddock's attempt to take the French Fort Duquesne, the English Army withdrew from western Pennsylvania. Confronting chaos in the western portions of the colony, Pennsylvania called up the militia and in March 1756, Mercer accepted a commission as captain. Mercer took part in a raid against the Shawnee and Delaware Indians in Kittanning on September 8, 1756. Mercer was wounded severely in the raid, and was detached from his men. Badly wounded, he walked one hundred miles for medical attention. The following account of his journey in the Pennsylvania Gazette in 1756 brought him fame in the colony:

We hear that Captain Mercer was 14 Days in getting to Fort Littleton. He had a miraculous Escape, living ten Days on two dried Clams and a Rattle Snake, with the Assistance of a few Berries. The Snake kept sweet for several Days, and, coming near Fort Shirley, he found a Piece of dry Beef, which our People had lost, and on Trial rejected it, because the Snake was better. His wounded Arm is in a good Way, tho' it could be but badly drest, and a Bone broken.

The city of Philadelphia would award him a Silver Medal in commemoration of his bravery.

In the next four years of his life, Mercer rose in the ranks of the Army and met George Washington. After his injury, Mercer moved to Fredericksburg, Virginia where he met his wife Isabella Gordon. Together Mercer and his wife had five children. During his 14 years in Virginia, he operated a thriving medical and apothecary practice. Paula Felder quotes an English visitor at the time who described Mercer as "a physician of great merit and eminence, and as a man, possessed of almost every virtue and accomplishment of a just and moderate way of thinking, and a generosity of principle."

An ardent patriot, Mercer joined Washington's Army as colonel of the third Virginia Regiment in 1775. Under his command, Mercer had future President James Monroe and future Chief Justice John Marshall. In 1776, the Continental Congress later appointed Mercer as Brigadier General to construct Fort Lee on the New Jersey side of the Hudson River. Despite the preparations, the Continental Army was force to retreat across New Jersey. At the end of that year, Mercer would be prominent in the Christmas attack on the Hessians in Trenton, New Jersey and the famous crossing of the Delaware River.

In an effort to follow up on the victory at Trenton, General Washington wanted to attack the British from behind and capture Princeton. On January 3, 1777, Mercer and his men were leading the wing of the press forward, encountering the British first in the middle of an orchard. The British charged and forced the American troops out of the orchard, resulting in the shooting of Mercer's horse. Mercer was separated from his men and quickly caught by the British. Mercer was asked to surrender but refused and charged with his sword drawn. He was beaten with muskets and stabbed by several bayonets in the assault. Washington saw that Mercer's men were retreating and he brought reinforcements. Mercer was found and taken to the Thomas Clarke House for medical attention.

He was treated by the famed Philadelphia physician, Dr. Benjamin Rush, but died nine days later on January 12, 1777. His body was taken to Philadelphia, at the Christ Church Burial Ground. In 1840, the local Saint Andrew's Society (a Scottish fraternal group) moved his body to its current location, Philadelphia's Laurel Hill Cemetery.

Mercer's life and heroic death was inspirational to many in the early years of the Republic. Counties in Illinois, Kentucky, New Jersey, Ohio, and West Virginia were named in honor of the General. In the Commonwealth of Pennsylvania, Mercer County was named for him at the time of its creation in 1800 and the town of Mercersburg was also named in his honor.