Along with Meriwether Lewis, Clark helped lead the Lewis and Clark Expedition of 1804 to 1806 across the Louisiana Purchase to the Pacific Ocean, and claimed the Pacific Northwest for the United States. Before the expedition, he served in a militia and the United States Army. Afterward, he served in a militia and as governor of the Missouri Territory. From 1822 until his death in 1838, he served as Superintendent of Indian Affairs.
William Clark was born in Caroline County, Virginia, on August 1, 1770, the ninth of ten children of John and Ann Rogers Clark. His parents were natives of King and Queen County, and were of English and possibly Scots ancestry. The Clarks were planters in Virginia and owned several modest estates and a few slaves. They were members of the Anglican Church.
Clark did not have any formal education; like many of his contemporaries, he was tutored at home. In later years, he was self-conscious about his convoluted grammar and inconsistent spelling—he spelled "Sioux" 27 different ways in his journals of the Lewis and Clark Expedition—and sought to have his journals corrected before publication. The spelling of American English was not standardized in Clark's youth, but his vocabulary suggests he was well read.
Clark's five older brothers fought in Virginia units during the American Revolutionary War (1775–1783), but William was too young. His oldest brother, Jonathan Clark, served as a colonel during the war, rising to the rank of brigadier general in the Virginia militia years afterward. His second-oldest brother, George Rogers Clark, rose to the rank of general, spending most of the war in Kentucky fighting against British-allied American Indians. After the war, the two oldest Clark brothers made arrangements for their parents and family to relocate to Kentucky.
William, his parents, his three sisters, and the Clark family's slaves arrived in Kentucky in March 1785, having first traveled overland to Redstone Landing in present-day Brownsville, Pennsylvania. They completed the journey down the Ohio River by flatboat. The Clark family settled at "Mulberry Hill", a plantation along Beargrass Creek near Louisville. This was William Clark's primary home until 1803. In Kentucky, his older brother George Rogers Clark taught William wilderness survival skills.