John Sullivan moved from legal practice in New Hampshire into Revolutionary politics and military command. He represented the colony in the Continental Congress and became a brigadier general in the Continental Army in 1775. During the failed invasion of Canada in 1776, he assumed responsibility for troops retreating after disease and military reverses had damaged the campaign. Promoted to major general, he was captured at the Battle of Long Island later that year.

After an exchange he returned to Washington's army and commanded a division in the victory at Trenton. His subsequent service included the unsuccessful attack on Staten Island and the battles of Brandywine and Germantown in 1777. These operations brought criticism and disputes, alongside independent command. In 1778 he directed the American operation against Newport, Rhode Island, which failed to capture the British position after cooperation with the French fleet broke down.

In 1779 Washington ordered Sullivan to lead an expedition against British-allied Haudenosaunee communities in New York and Pennsylvania. Joined by troops under James Clinton, his army won at Newtown and destroyed towns, homes, orchards, and food supplies. The campaign aimed to break resistance and support for Britain, but its consequences extended beyond combat: displaced families faced hunger, exposure, and disease. Military success against settlements did not produce an uncomplicated or permanent security settlement on the frontier.

Sullivan left the army in 1779 and resumed civil office. He returned to Congress, served as New Hampshire's attorney general, and held the state's chief executive office, then called president, in the 1780s. He chaired the convention that ratified the United States Constitution in 1788. Washington appointed him the first federal district judge for New Hampshire in 1789, a position he held until his death in 1795. His career joined institution-building to destructive frontier warfare.