John Langdon (b. June 26, 1741; d. September 18, 1819) was a Governor and United States Senator from New Hampshire and the first President pro tempore of the Senate. He was also a member of the Continental Congress and a signer of the United States Constitution.
Langdon was born in Portsmouth, New Hampshire, on June 26, 1741. His father was a prosperous landowner; his older brother, Woodbury Langdon, would also go on to serve in the Continental Congress later in life. John attended the local grammar school, and later served an apprenticeship as a clerk.  Over the Rather than follow his father into farming, he went to sea, and engaged in mercantile pursuits, becoming captain of his own ship by the age of 22. Over the next several years, he built a small fleet of vessels, becoming one of Portsmouth's wealthiest citizens. 
Langdon was elected to the New Hampshire legislature in 1774 on a platform of strong opposition to British taxation and support for the Continental Association's boycott of British goods. (Ironically, his brother was also elected to the legislature the same year as a voice for merchants who opposed the boycott.) Recognizing that more direct action would be needed, John Langdon joined a group of militiamen in a raid to remove the gunpowder from the local fort before it could be seized by the Crown.
Continental Congress and Constitutional Convention
In 1775, on the heels of the gunpowder raid, Langdon was appointed to the Continental Congress. He immediately became a strong voice for independence. He served through 1776, at which time he resigned to take a more direct hand in the war effort and in outfitting ships of war. He was once again appointed to the Continental Congress in 1787, and served as one of New Hampshire's delegates to the 1787 Constitutional Convention in Philadelphia.
Later Political Service
Langdon was elected to the United States Senate, one of New Hampshire's first Senators. He served from March 4, 1789, to March 3, 1801. On April 6, 1789, he was elected the first President pro tempore of the Senate, presiding over the electoral vote that made George Washington the first President of the United States. Langdon also served as President pro tempore of the Senate during the Second Congress.
Retiring from the Senate in 1801, Langdon rejected President Thomas Jefferson's offer to become Secretary of the Navy. Instead he returned to his business interests in New Hampshire. He continued to lead an active political life, both as legislator and governor, until retiring in 1812--declining a nomination as candidate for Vice President of the United States. 
John Langdon died in Portsmouth, New Hampshire on September 18, 1819; he was interred in the family tomb in the North Cemetery.