After receiving news of the battle of Lexington and Concord, Alexander Hamilton began to prepare for war. With his old friend Robert Troup and other King's College men, Hamilton formed a volunteer musket drill unit and, with the assistance of some of his veteran professors, trained a group of young college boys to shoot and fight. Hamilton also began studying artillery with the help of his math professor. He and his volunteers saw their first military action on the night of August 23, 1775, when they helped capture twenty-one cannon at a British stockade on the tip of Manhattan Island in New York. The operation was not overly dangerous, because the stockade was only protected by one warship, which could do little to deter the land-based raid. Nonetheless, Hamilton achieved some measure of glory, and proved his mettle on the battlefield.

By March of 1776, Hamilton had been commissioned a captain in the Continental Army and given command of an artillery company of nearly 100 men. As a gunnery captain, Hamilton demonstrated superior command of administrative and logistical matters; his men were always well fed and well paid, his guns were always properly maintained, and his supplies never ran low. Hamilton's company participated in many skirmishes in New York in the autumn of 1776, and served as part of the attacking force at the Battle of Princeton in January of 1777.

Hamilton's valor and of organization brought him to the attention of General George Washington, Commander-in-Chief of the Continental Army and the militias. Immediately following the Battle of Princeton, Washington invited Hamilton to become one of his aides. Hamilton accepted the offer, and was promoted to the rank of lieutenant-colonel at the age of twenty-one. He remained on Washington's staff for four years. His primary duties included drafting letters for the General addressed to congressmen, governors, and other supporters, and he also coordinated supplies and munitions movements. Hamilton earned the affectionate nickname the "Little Lion" because of his lean stature and intelligence, and Washington himself grew fond of the young lieutenant- colonel, who he came to rely on heavily. Hamilton did not participate in any military engagements while on Washington's staff, he did suffer with the troops at the Battle of Philadelphia, and was at Germantown when the British smashed the Continental Army. Hamilton also stayed with Washington at Valley Forge during the bitterly cold and disastrous winter of 1777–1778.

Hamilton assisted Washington when the British later evacuated Philadelphia in the spring of 1778, and at the Battle of Monmouth in June of the same year. At Monmouth, Hamilton had his horse shot out from under him, although he himself was unharmed. After the battle, the American General Charles Lee was court- martialed for misconduct during the fight. Lieutenant-colonel Hamilton testified against Lee, as did another of Washington's aides, Colonel John Laurens. After the court-martial, Hamilton then served as Laurens's second in a duel between Lee and Laurens.

After Monmouth, Hamilton was assigned to tracking down the infamous Benedict Arnold, but chasing criminals was hardly enough to satisfy Hamilton. Still, his time with Washington proved to be the most educational experience of Hamilton's life. He developed the leadership skills necessary to life as a public figure, as well as forming a strong and lasting friendship with George Washington. This friendship helped Hamilton solidify his political beliefs. But not everything about Hamilton's life in the military was hard work. As a young and dashingly handsome colonel, Hamilton and Washington's other assistants often visited many eligible young ladies. For years, Hamilton openly denied he would ever marry, but he eventually fell in love with Elizabeth Schuyler, whom he married on December 14, 1780. Betsey was the daughter of the American general Philip Schuyler, which brought Hamilton both military connections and wealth, but most historians now believe that Hamilton did indeed love his wife very much.

In July of 1881, Washington gave Hamilton command of an infantry battalion in New York. Hamilton and his men fought bravely at the Battle of Yorktown in October of the same year. Armed with a musket and bayonet, Hamilton ran ahead of his battalion to engage the British, who eventually retreated. Soon after Yorktown, Hamilton left the army to return home to his wife, who was by this time pregnant with their first child.