Gridley sided with the Thirteen Colonies during the American Revolutionary War and was made Chief Engineer in the New England Provincial Army. He laid out the defenses on Breed's Hill and was wounded at the Battle of Bunker Hill. When the Continental Congress first created a Continental Army under command of George Washington in 1775 Gridley was named to Chief Engineer (artillery). He directed the construction of the fortifications on Dorchester Heights which forced the British to evacuate Boston in March 1776. When Washington moved his army south, Gridley remained as Chief Engineer of the Eastern Department.