Stark served as a second lieutenant under Major Robert Rogersduring the French and Indian War. His brother William Starkserved beside him. As a member of the daring Rogers' Rangers, Stark gained valuable combat experience and a detailed knowledge of the northern frontier of the American colonies.

Stark's action contributed to the surrender of Burgoyne's northern army after the Battles of Saratoga by raising American morale, by keeping the British from getting supplies, and by subtracting several hundred men from the enemy order of battle. Stark reported 14 killed and 42 wounded. Of Baum's 374 professional soldiers, only nine men escaped. For this feat Stark won his coveted promotion to brigadier general in the Continental Army on October 4, 1777. Saratoga is seen as the turning point in the Revolutionary War, as it was the first major defeat of a British general and it convinced the French that the Americans were worthy of military aid. After the Battle of Freeman's Farm Gen. Stark's brigade moved into a position at Stark's Knob cutting off Burgoyne's retreat to Lake George and Lake Champlain.

John Stark sat as a judge in the court martial that in September 1780 found British Major John André guilty of spying and in helping in the conspiracy of Benedict Arnold to surrender West Point to the British. He was the commander of the Northern Department three times between 1778 and 1781 along with commanding a brigade at the Battle of Springfield in June 1780.