Langdon was an American statesman born in Portsmouth, New Hampshire. His position as a wealthy ship owner and merchant made him valuable to the colonists as a leader in their struggle against Great Britain. He was a member of the Continental Congress in 1775 and 1776. In the latter year he left Congress to act as naval agent for the government, supervising the construction of warships. In 1777 he financed, organized, and served in the brigade led by General John Stark against the British forces under General John Burgoyne. He again served as a member of the Continental Congress in 1783. He became president of New Hampshire in 1785 and governor of the state in 1788. From 1789 to 1801 he represented New Hampshire in the U.S. Senate. From 1805 to 1809 and from 1810 to 1812 he was again governor of New Hampshire.