There are rumors that Mercer exclusively originated Washington's plan to cross the Delaware River and surprise the Hessians at the Battle of Trenton on December 26, 1776, and he was certainly a major contributor to its execution. Due to the victory at Trenton (and a small monetary bonus), Washington's men agreed 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.