William Clark, of the Corps of Discovery, was born in the same area of Virginia that was home to his co-captain, Meriwether Lewis. Clark learned about wilderness skills and natural history from his older brother and at the age of 19 began a military career in the Kentucky Militia. He later joined the regular army and was promoted to lieutenant. Ensign Meriwether Lewis was among men assigned to Clark. The two struck up a lasting friendship that would lead to their co-commanding the Corps of Discovery.

The Louisiana Purchase in 1803 led to the need to explore, document, and consolidate the claim of the United States to the vast territory. President Thomas Jefferson groomed Meriwether Lewis to lead the expedition and Lewis sought out Clark to share the leadership. While less educated than Lewis, Clark was an excellent cartographer. His maps from the expedition included notes on a variety of subjects such as native botanical and zoological specimens and potential mineral deposits. The party spent a long and wet winter in the general vicinity of the mouth of the Columbia River in Oregon before returning east. The accomplishments of the Corps of Discovery played a fundamental role in understanding the nature and inhabitants of the West and laid the groundwork for future expansion.

As a reward for his service, President Jefferson awarded Clark 1,600 acres of public land. In 1813, he was named Governor of the Missouri Territory until the state of Missouri was created in 1820. Although he was defeated in the first election for state governor, Clark continued to hold his Brigadier General of Militia rank, and to serve as the Superintendent of Indian Affairs. Throughout the remainder of his life, he retained the respect of Native Americans, traders, and trappers. They regularly brought new information to him that he used to update his master map of the West, a map that graphically displayed the fast-changing face of an expanding nation. Clark died of natural causes in St. Louis on September l, 1838.

Commemorating the bicentennial of the 1804-1806 expedition, celebrations began in 2004 in cities and historic sites along the Corps of Discovery's route.