In 1775, Montgomery was one of the signers of the Fincastle Resolutions, the earliest statement of armed resistance to the British Crown in the American Colonies. During the American Revolutionary War, Montgomery served initially as a captain and finally as a lieutenant colonel under the command of Brigadier General George Rogers Clark during his Illinois campaign. Under Clark's orders, Montgomery led an expedition against several Indian tribes to prevent the British from organizing them to attack Kentucky. For the remainder of war, he ran supplies between New Orleans and Clark's army in the north.