Donop split his forces into two groups, one group was to attack the southern side of the fort, while the other took on the northern and eastern sides, along with six British Men-of-war ships to help support them. With such a strong strategy in hand, Donop was convinced that the fort would be his, but it was not to be. Even after launching a cannonade on the fort's southern side, the Hessian forces were cut down by cannon and musket fire on their approach. The forces on the northern side had better success, scaling the ramparts on a abandoned part of the fort, but were met with tangles of trees and plants that impeded their progress. They were soon spotted by American forces and were forced to retreat back to their camp. The British ships fared no better, even against smaller American gunboats, with two ships (The Augusta and the Merlin) running aground after trying to avoid underwater obstacles.
At the end of the battle, around 380 Hessians were dead, while the Americans only suffered 14 deaths. von Donop was mortally wounded in the battle and died three days later. British General Sir William Howe was very frustrated about the failed capture and ordered the Hessian forces withdrawn from New Jersey while he focused on building his forces for an attempted capture of Fort Mifflin, which was successful after a five day bombardment. Howe then sent Lord Cornwallis with over 2,000 British troops to once again attempt to capture Fort Mercer. But they found the fort abandoned, not knowing that the fort's commander, Colonel Christopher Greene, had decided to pull his troops out of the fort, rather than risk their lives in a British Assault.