John Langdon (June 26, 1741 - September 18, 1819) was an American politician and the first President pro tempore of the United States Senate.
Born in Portsmouth, New Hampshire, Langdon attended the local grammar school, served an apprenticeship as a clerk, went to sea, and engaged in mercantile pursuits. Langdon was a prominent supporter of the revolutionary movement and active in the American Revolutionary War.
He served as a member of the Continental Congress in 1775 and 1776. He resigned in June 1776 to become agent for the Continental forces against the British and superintended the construction of several ships of war. In 1777, he equipped an expedition against the British, participating in the Battle of Bennington and commanding a company at Saratoga and in Rhode Island.
Langdon was again a member of the Continental Congress in 1787 and became a delegate to the Federal Constitutional Convention in 1787, serving as a member of the New Hampshire ratifying convention. He was elected to the United States Senate and served from March 4, 1789, to March 3, 1801. He was elected the first President pro tempore of the Senate on April 6, 1789. He also served as President pro tempore during the Second Congress.
He later served as a member of the New Hampshire legislature from 1801-05, the last two terms as speaker. He was Governor of New Hampshire from 1805-11, with the exception of 1809. He declined the nomination to be a candidate for vice president in 1812.
Langdon died in his home town of Portsmouth in 1819. Interment is in the Langdon tomb in North Cemetery in Portsmouth.