Born at Pitsligo on the 16 January, 1726, a son of the manse, Hugh Mercer started his studies in medicine at Marischal College in1740. Graduating with an MA in1744, he apprenticed as a young doctor with a Dr. Cruickshank from Ellon. He became assistant surgeon in Bonnie Prince Charlie's Army during the Jacobite rebellion where he was a surgeon’s mate at the Battle of Culloden in 1746. After the rout, he made his way towards the North-East but was followed by English troops. Travelling to Leith, Mercer joined a ship heading for Philadelphia where in a nearby town, he practiced medicine for the next eight years. There he witnessed the atrocities occurring in the French and Indian War. So upset, he became a soldier in the British Colonist Army, reaching the rank of Colonel. Another Colonel, George Washington became his best friend, fighting numerous campaigns together and he was to become a major strategist and military leader during the War of Independence. Mercer was wounded at Kittanning, Ohio, receiving a medal for his services in the 7-year war. On the advice of George Washington, he re-started his medical practice in Fredericksburg, Virginia.

He was called to arms again to the cause of the Americans and the drive for independence. It was suggested that Mercer exclusively planned Washington’s famous “Crossing of the Delaware” to surprise the Hessians at the Battle of Trenton on 26 December 1776. Promoted to General, Mercer supported the future President on his retreat through New Jersey. Mercer was killed at the Battle of Princeton on 12 January 1777 having been shot and bayoneted to death for refusing to surrender. However, he had led his men to an important battle which helped turn the tide against the British. His efforts contributed to Washington's ultimate victory. It has been argued by historians that Mercer was a better leader and soldier than Washington.

A grandson became a governor of Virginia and General George Patton is a descendent.

Mercere is remembered in America by many places being named after him. He is commemorated by a plaque in the Quadrangle, Marischal College, Aberdeen.