The county was named for Continental Army General Hugh Mercer, who died as a result of wounds received at the Battle of Princeton on January 3, 1777.

A Scotsman that fled to British North America after the failed Jacobite Rebellion, he worked closely with George Washington in the American Revolution.

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

Getting to his feet, he was quickly surrounded by British troops who mistook him for George Washington and ordered him to surrender.

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.

The Mercer Oak, against which the dying general rested as his men continued to fight, appears on the county seal and stood for 250 years until it collapsed in 2000.

After the Legislature relocated to Trenton from Perth Amboy in 1790, it purchased land for £250 and 5 shillings and constructed a new state house, designed by Philadelphia-based architect Jonathan Doane, beginning in 1792.

The Doane building was covered in stucco, measured 150 by 50 feet (46 by 15 m), and housed the Senate and Assembly chambers in opposite wings.

Mercer County was the landing spot for a fictional Martian invasion of the United States.

In 1938, in what has become one of the most famous American radio plays of all time, Orson Welles acted out his The War of the Worlds invasion.

Further northwest, terrain rises as it ascends the Piedmont Plateau, with The Sourlands encompassing the far northwestern portion of the county.

Baldpate Mountain, in the western part of Hopewell Township, is the highest point in the county, at 480 feet (150 m) above sea level.

[28] The lowest elevation is sea level, spread out along the shores of the tidal portions of the Delaware River, Crosswicks Creek and the smaller waterways within the Trenton-Hamilton Marsh in Hamilton Township and the city of Trenton.

[16] Based on data from the Bureau of Economic Analysis, the county had a gross domestic product of $36.0 billion in 2021, ranked seventh in the state and a 6.7% increase from the prior year.

[39] Trenton's role as New Jersey's state capital contributes significantly to Mercer County's economic standing.

The Board is led by a Commissioner Chair and vice-chair, selected from among its members at an annual reorganization meeting held in January.

The Commissioner Board votes to approve the budget prepared by the Executive after review and modifications are made.

[45] As of 2025[update], the Mercer County Executive is Daniel R. Benson (D, Hamilton Township) whose term of office ends December 31, 2027.

[46] Mercer County's Commissioners are (with terms for chair and vice chair ending every December 31): Pursuant to Article VII Section II of the New Jersey State Constitution, each county in New Jersey is required to have three elected administrative officials known as "constitutional officers."

[66] The vicinage is seated at the Mercer County Criminal Courthouse, located at 400 South Warren Street in Trenton.

[67] For the 119th United States Congress, New Jersey's 3rd congressional district is currently represented Herb Conaway (D, Delran Township).

[68] For the 119th United States Congress, New Jersey's 12th congressional district is represented by Bonnie Watson Coleman (D, Ewing Township).

The New Jersey Turnpike (I-95) passes through the southeastern section of the county, and serves as a major corridor to Delaware, Washington, D.C. to the south, and New York City and New England towards the north.

Long-distance transportation is provided by Amtrak train service along the Northeast Corridor through the Trenton Transit Center.

[82] The 12 municipalities in Mercer County (with 2010 Census data for population, housing units and area) are:[83] Allens Station, Eiler Corner, Etra, Hickory Corner, Locust Corner, Millstone, Twin Rivers CDP (7,787) Berrien City, Clarksville, Dutch Neck, Edinburg, Edinburg Park, Golf View Manor, Grover's Mill, Old Mill Farms, Penns Neck, Port Mercer, Post Corner, Princeton Colonial Park, Princeton Estates, Princeton Ivy East, Princeton Junction CDP (2,475), Sherbrook Estates Mercer County has a number of large parks.

The largest, Mercer County Park is the home for the US Olympic Rowing Team's training center.

The minor league hockey team, the Trenton Titans, established in 1999 and operating as the ECHL affiliate of the NHL's Philadelphia Flyers and the AHL's Adirondack Phantoms, disbanded before the start of the 2013–14 season.

[87] Mercer County is also home to several college athletic programs, including two NCAA DI schools.