The Battle of Iron Works Hill, also known as the Battle of Mount Holly, was a series of minor skirmishes that took place on December 22 and 23, 1776, during the American Revolutionary War. The fighting took place in Mount Holly, New Jersey, between an American force mostly composed of colonial militia under Colonel Samuel Griffin and a force of 2,000 Hessians and British regulars under Carl von Donop. While the American force of 600 was eventually forced from their positions by the larger Hessian force, the action prevented von Donop from being in his assigned base at Bordentown, New Jersey, and in a position to assist Johann Rall's brigade in Trenton, New Jersey, when it was attacked and defeated by George Washington after his troops crossed the Delaware on the night of December 25–26, 1776. As the troops of von Donop and Rall occupied the last outposts, they were often exposed to the actions of rebel raids and the actions of Patriot militia forces that either arose spontaneously or were recruited by Army regulars. These actions frayed the nerves of the troops, as the uncertainty of when and where such attacks would take place, and by what size force, put the men and their commanders on edge, leading them to jump up to investigate every rumored movement.

On December 21, about 600 of Griffin's troops overwhelmed a guard outpost of the 42nd located about one mile south of Blackhorse at Petticoat Bridge. On the evening of December 22, Washington's adjutant, Joseph Reed, went to Mount Holly and met with Griffin. Griffin had written to Reed, requesting small field pieces to assist in their actions, and Reed, who had been discussing a planned attack on Rall's men in Trenton with Washington, wanted to see if Griffin's company could participate in some sort of diversionary attack. Griffin was ill, and his men poorly equipped for significant action, but they apparently agreed to some sort of actions the next day. On December 23, Von Donop's forces bivouacked in Mount Holly during the night, where, according to Ewald, they plundered the town, breaking into alcohol stores of abandoned houses and getting drunk. Von Donop himself took quarters in the house that Ewald described as belonging to an "exceedingly beautiful widow of a doctor", whose identity is uncertain. On December 24, they moved in force to drive the militia from the hill, but Griffin and his men had retreated to Moorestown during the night. For whatever reason, von Donop and his contingents remained in Mount Holly, 18 miles and a full day's march from Trenton, until a messenger arrived on December 26, bringing the news of Rall's defeat by Washington that morning.