He served as an assistant surgeon in Charles Edward Stuart's army during the Battle of Culloden in the Jacobite rising of 1745.

[9] In 1755, Mercer served as a captain in General Edward Braddock's army in his failed attempt to take Fort Duquesne.

He trekked 100 miles (160 km) through the woods for 14 days, injured and with no supplies, before he "lay down, giving up all hopes of ever getting home."

Washington, who was also a member of this lodge, later became president, and at least eight members were generals of the Continental Army during the Revolutionary War: Washington, Mercer, George Weedon, William Woodford, Fielding Lewis, Thomas Posey, Gustavus Wallace, and the Marquis de Lafayette, who named an honorary general in 1824, far more than any other group, institution, or organization during the Revolutionary War.

[22] In 1774, George Washington sold Ferry Farm, his childhood home, to Mercer, who wanted to make this prized land into a town where he and his family would settle for the remainder of his days.

On June 5, 1776, Mercer received a letter from the Second Continental Congress in Philadelphia, signed by John Hancock, appointing him brigadier-general in the Armies of the United Colonies and requesting him to report to headquarters in New York immediately.

[32] Mercer led a raid on Richmondtown, Staten Island on October 15, 1776, temporarily securing the town and taking as prisoners those inside the makeshift hospital of St. Andrew's Church, but was repelled back to New Jersey, releasing the prisoners and causing numerous British casualties in the process.

[33] Some historical accounts credit Mercer with the suggestion for George Washington's crossing of the Delaware River, which resulted in a surprise attack on the Hessians at the Battle of Trenton on December 26, 1776.

[34] The victory at Trenton (and a small monetary bonus) made Washington's men agree to a ten-day extension to their enlistment.

When Washington decided to face off with Cornwallis during the Second Battle of Trenton on January 2, 1777, Mercer was given a major role in the defense of the city.

While leading a vanguard of 350 soldiers, Mercer's brigade encountered two British regiments and a mounted unit.

Legend has it that a beaten Mercer, with a bayonet still impaled in him, did not want to leave his men and the battle and was given a place to rest on a white oak tree's trunk, and those who remained with him stood their ground.

Medical efforts were made by Rush to save Mercer,[38] but he was mortally wounded and died nine days later, on January 12, 1777.

In 1840, he was reinterred in Laurel Hill Cemetery,[39] including a memorial monument funded by the Saint Andrew's Society.

The crisis ended, demonstrating that Washington and his army had the means to fight, and British public support for continued engagement in the war began waning.

[41] A second portrait by Charles Willson Peale, Washington at the Battle of Princeton, January 3, 1777, displays Washington in the foreground with Hugh Mercer lying mortally wounded in the background, supported by Dr. Benjamin Rush and Major George Lewis holding the American flag.

Peale painted a version of Battle of Princeton, whose background shows a very indistinct portrait of Mercer being helped from the ground.