Born in 1783 in Connecticut, Henry Leavenworth was educated in Vermont and New York. He became a lawyer and entered the military. He made his way up the ranks, eventually declared colonel of the 25th U. S. infantry. He then served in the New York State Assembly, and in 1816, went to Prairie du Chien as Indian agent. In 1818, he was made lieutenant-colonel of the Fifth U. S. infantry.
In 1805 the U.S. Army ordered Lt. Zebulon Pike to explore the Mississippi River and select potential sites for future military posts. When he arrived at the junction of the Mississippi and St. Peters (the present-day Minnesota) rivers, Pike made a treaty with several local Dakota representatives and acquired land on which he promised a U.S. trading post would be built. The trading post was never constructed, but following the War of 1812 the U.S. government sought to firmly establish its presence and stamp out any existing British influence in the Northwest Territory by building a fort at the river junction. The first troops arrived in 1819 under the command of Leavenworth. In 1820 he initiated the construction of Fort St. Anthony, which was later called Fort Snelling.