Jefferson and Lewis both realized that the expedition would need someone to share in the leadership duties. Lewis chose William Clark, whom Lewis had served under for six months while on army duty from 1795 to 1796. Clark was also originally from Virginia, the ninth of ten children born to a successful planter. His brother was the famous Revolutionary War veteran General George Rogers Clark.
William Clark had retired from the army in 1796, and was living at home in Clarksville, Indiana Territory, when he received Lewis's invitation to join the expedition. Lewis described the plans for the expedition, summarized Jefferson's instructions, and asked that Clark, if he accepted, begin finding other men to hire for the journey. On July 29, 1803, Lewis received Clark's answer. Clark eagerly accepted Lewis's proposal, writing that
"I will chearfully join you in an 'official Charrector' as mentioned in your letter, and partake of the dangers, difficulties, and fatigues, and I anticipate the honors & rewards of the result of such an enterprise. . . . This is an undertaking [fraught] with many difeculties, but My friend I do assure you that no man lives with whome I would [prefer] to undertake Such a Trip [etc.] as yourself."
Lewis responded on August 3, happily acknowledging Clark's acceptance. On October 15, Lewis arrived at Clark's home in Clarksville, where Clark and his slave, York, joined the expedition. Lewis knew that William Clark would be a great asset to the expedition. Clark had been a company commander, and was therefore used to leadership. He was a better surveyor than Lewis and had strong mapmaking abilities, as well as solid forestry skills. Although the two men did not know each other very well at the time the expedition was being organized, Lewis and Clark both liked and trusted each other. Together, the two men led an expedition that saw only one casualty (and that from an illness they could not have prevented), and that proved of immense value to the rapidly forming United States.
After the expedition, Clark was named governor of the Missouri Territory — a position which lasted until the territory became a state in 1920. Due to Lewis's death, Clark ultimately oversaw the publication of their journals in 1814, using Clark's new map as a supplement. Clark, appointed by Jefferson as Superintendent of Indian Affairs, also devoted much of his life to very successful negotiations with and between various Native American tribes. William Clark died at the age of 69, of natural causes, on September 1, 1838 in St. Louis, Missouri.
