earborn was born in Hampton, New Hampshire. He studied medicine and opened practice in 1771. When Revolutionary fighting began, he led a local militia troop to Boston and fought at Bunker Hill. He then served under Benedict Arnold during the American expedition to Quebec, where he was captured and imprisoned for a year. Dearborn later fought at Ticonderoga, Monmouth, and in the Genesee Valley. He also served on George Washington's staff at Yorktown. After the war, he worked as U.S. marshal for the District of Maine. In 1793, he represented Massachusetts in Congress for two terms. In 1801, President Thomas Jefferson appointed him Secretary of War, a post he held for eight years.

uring the War of 1812, Dearborn commanded the Army's northeast sector. He lost Detroit and several other forts in the Great Lakes to the British while sustaining heavy casualties in the capture of Toronto. As a result, he was reassigned to an administrative command in New York City. In 1815, President James Madison recommended Dearborn's reappointment as Secretary of War. The Senate rejected the nomination. He later served two years as U.S. Minister to Portugal.