A brilliant law student, Burr interrupted his study to serve in the American Revolution and proved himself a valiant soldier in early campaigns. In 1779 ill health forced him to leave the army. Upon admission (1782) to the bar, he plunged energetically into the practice of both law and politics. He served as a member (1784–85; 1797–99) of the New York assembly, as state attorney general (1789–91), and as U.S. Senator (1791–97).
Defeated for reelection to the assembly in 1799, he set about organizing the Republican (see Democratic party) element in New York City for the election of 1800, for the first time making use of the Tammany Society for political purposes. The result was an unexpected victory for the Republicans, who gained control of the state legislature. Since the legislature named presidential electors and New York was the pivotal state, Burr's victory insured the election of a Republican president.
The intention of the party was to make Thomas Jefferson president and Burr vice president, but confusion in the electoral college resulted in a tie vote. This threw the election into the House of Representatives. There, the Federalist Alexander Hamilton, who regarded Jefferson as the lesser evil of the two Republicans, helped to secure Jefferson the presidency, and on the 36th ballot Burr became vice president.
Burr presided over the Senate with a dignity and impartiality that commanded respect from both sides, and in 1804 his friends nominated him for the governorship of New York. Hamilton again contributed to his defeat, in part by statements reflecting on Burr's character. Burr challenged Hamilton to a duel and mortally wounded him.