Aaron Burr Jr. was born in Newark, New Jersey, in 1756 as the second child of the Reverend Aaron Burr Sr., a Presbyterian minister and second president of the College of New Jersey, which became Princeton University. His mother Esther Edwards Burr was the daughter of noted theologian Jonathan Edwards and his wife Sarah. Burr had an older sister Sarah ("Sally") who was named for her maternal grandmother. She married Tapping Reeve, founder of the Litchfield Law School in Litchfield, Connecticut.
Burr's father died in 1757, and his mother died the following year, leaving him and his sister orphans when he was two years old. He and his sister first lived with their maternal grandparents, but his grandmother also died in 1757, and his grandfather Jonathan Edwards died in 1758. Young Aaron and Sally were placed with the William Shippen family in Philadelphia. In 1759, the children's guardianship was assumed by their 21-year-old maternal uncle Timothy Edwards. The next year, Edwards married Rhoda Ogden and moved with the children to Elizabeth, New Jersey near her family. Burr had a very strained relationship with his uncle, who frequently employed physical punishment. As a child, he made several attempts to run away from home.
Burr was admitted to Princeton as a sophomore at age 13 where he joined the American Whig Society and the Cliosophic Society, the college's literary and debating societies. He received his Bachelor of Arts degree in 1772 at age 16 but continued studying theology at Princeton for an additional year. He then undertook rigorous theological training with Joseph Bellamy, a Presbyterian, but changed his career path after two years. At age 19, he moved to Connecticut to study law with his brother-in-law Tapping Reeve, who had married Burr's sister in 1771. News reached Litchfield in 1775 of the clashes with British troops at Lexington and Concord, and Burr put his studies on hold to enlist in the Continental Army.
During the American Revolutionary War, Burr took part in Colonel Benedict Arnold's expedition to Quebec, an arduous trek of more than 300 miles (480 km) through the frontier of Maine. Arnold was deeply impressed by Burr's "great spirit and resolution" during the long march, and he sent Burr up the Saint Lawrence River when they reached Quebec City to contact General Richard Montgomery, who had taken Montreal, and escort him to Quebec. Montgomery then promoted Burr to captain and made him an aide-de-camp. Burr distinguished himself during the Battle of Quebec on December 31, 1775, where he attempted to recover Montgomery's corpse after the General had been shot.
In the spring of 1776, Burr's stepbrother Matthias Ogden helped him to secure a place on George Washington's staff in Manhattan, but he quit within two weeks on June 26 to be on the battlefield; there was more honor to be gained in battle than in the "insular world of the commander's staff," according to historian Nancy Isenberg. General Israel Putnam took Burr under his wing, and Burr saved an entire brigade from capture after the British landing on Manhattan by his vigilance in the retreat from lower Manhattan to Harlem. Washington failed to commend his actions in the next day's General Orders, however, which was the fastest way to obtain a promotion. Burr was already a nationally known hero, but he never received a commendation. According to Ogden, he was infuriated by the incident, which may have led to the eventual estrangement between him and Washington. Nevertheless, Burr defended Washington's decision to evacuate New York as "a necessary consequence." It was not until the 1790s that the two men found themselves on opposite sides in politics.
Burr was promoted to lieutenant colonel in July 1777 and assumed virtual leadership of Malcolm's Additional Continental Regiment. There were approximately 300 men under Colonel William Malcolm's nominal command, but Malcolm was frequently called upon to perform other duties, leaving Burr in charge. The regiment successfully fought off many nighttime raids into central New Jersey by Manhattan-based British troops who arrived by water. Later that year, Burr commanded a small contingent during the harsh winter encampment at Valley Forge, guarding "the Gulph," an isolated pass that controlled one approach to the camp. He imposed discipline and defeated an attempted mutiny by some of the troops.
Burr's regiment was devastated by British artillery on June 28, 1778 at the Battle of Monmouth in New Jersey and he suffered heat stroke. In January 1779, he was assigned to Westchester County, New York in command of Malcolm's Regiment, a region between the British post at Kingsbridge, Bronx and that of the Americans about 15 miles (24 km) to the north. This district was part of the larger command of General Alexander McDougall, and there was much turbulence and plundering by lawless bands of civilians and by raiding parties of ill-disciplined soldiers from both armies.
In March 1779, due to continuing bad health, Burr resigned from the Continental Army. He renewed his study of law. Technically, he was no longer in the service, but he remained active in the war; he was assigned by General Washington to perform occasional intelligence missions for Continental generals, such as Arthur St. Clair. On July 5, 1779, he rallied a group of Yale students at New Haven, Connecticut, along with Captain James Hillhouse and the Second Connecticut Governor's Guards, in a skirmish with the British at the West River. The British advance was repulsed, forcing them to enter New Haven from Hamden, Connecticut.
Despite these activities, Burr finished his studies and was admitted to the bar at Albany, New York in 1782. He married that same year and began practicing law in New York City the following year, after the British evacuated the city. He and his wife lived for the next several years in a house on Wall Street in Lower Manhattan.