The Clark family traces their heritage back to a John Clark who emigrated from England to Virginia in the early seventeenth century. The early Clarks were farmers and became important land owners in Virginia. William Clark's father, John Clark and his mother Ann Rogers, moved from King and Queen County in tidewater Virginia to the newly established county of Albemarle on the Rivanna River in the western Piedmont region of the state in 1749. A neighbor, Peter Jefferson, had been among the region's very first settlers in 1837. Peter's son, Thomas, would later become president of the United States. John and Ann had inherited 410 acres from John's father and they established a small tobacco farm. Their first son, Jonathan, was born in 1750, and their second, George Rogers, in 1752. In 1752 English settlers, organized into the "Ohio Company" pushed across the Appalachian Mountains into the watershed of the Ohio River. At the time France claimed the Mississippi and it's tributaries and reacted quickly to what they perceived as an English invasion. After ejecting the settlers from the Ohio region, the French built a fort at the Forks of the Ohio (present day Pittsburgh). In 1754, the French, along with their Indian allies, forced Lieutenant George Washington to surrender Fort Necessity in Pennsylvania and triggered the French and Indian War (also known as the Seven Years War). In 1755 Major General Edward Braddock was ambushed by an Indian force near the Forks of the Ohio and suffered a serious defeat.
Settlers in the Virginia Piedmont area became concerned for their safety and some retreated to the east to avoid hostilities. John and Ann Clark moved their family back to Caroline County in tidewater Virginia to a small farm that John had inherited from an uncle. In the succeeding years John and Ann had eight more children. The ninth child, William, was born on August 1, 1770. Like most of their neighbors, the Clarks owned slaves and one of "Billy's" childhood playmates was named York. As the two boys matured their relationship changed from playmates to master and servant. The Treaty of Paris ended the French and Indian War in 1763 and the French gave up their claims to North America. At the same time Indian unrest intensified. In 1763 the Ottawa chief Pontiac led a fiercely fought assault on English settlers and King George III issued a proclamation that proscribed trans-Appalachian settlement. King George was ignored and settlement of the Ohio Valley continued virtually unchecked with many prominent Americans participating in various lucrative land schemes. In the spring of 1772 George Rogers Clark explored the Ohio River Valley and laid claim to a stretch of land about forty miles south of present day Wheeling, West Virginia. In 1774 he participated in a military campaign against the Shawnee and Delaware Indians as a captain in the Virginia militia. The campaign, known as Lord Dunsmore's War, lasted into 1775 and resulted in the Treaty of Camp Charlotte by which the Shawnee agreed to relinquish all claims south of the Ohio River. This treaty presumed Kentucky to be part of Virginia and opened it to Anglo-European settlement.
In 1775 Daniel Boone cut a road through the Cumberland Gap and established a settlement called Boonesborough. In April 1775 the opening battles of the American Revolution were fought at Lexington and Concord. Although the major battles of the revolution were fought further to the east, British agents encouraged Indian attacks on settlers in Kentucky and Major George Rogers Clark emerged as the leader of the militia in Virginia's newly formed Kentucky County. In 1778 and early 1779 then Lieutenant Colonel George Rogers Clark at the head of a small force managed to clear the Illinois country of British forces. In the spring of 1779 George Rogers Clark built Fort Jefferson at the junction of the Ohio and Mississippi Rivers and thwarted a British attack plan that threatened St. Louis. Clark was thus the first official American to visit St. Louis. He was warmly received by the primarily French oriented community as the town's savior from the hated English. After participating in Baron Friedrich von Steuben's battle with Benedict Arnold at Hood's Ferry on the James River George Rogers Clark was promoted to brigadier general. On September 3, 1783, the Treaty of Paris was signed ending the American Revolution. At this point in time all of the Clark men had participated in the revolution except the youngest - Billy. Although rumors of excessive drinking marred his reputation, George Rogers Clark emerged as a heralded hero of the western revolutionary campaigns. Billy Clark was thirteen years old and had an obvious role model.
In October 1783 John and Ann Clark moved to land that their son George Rogers Clark had claimed for them at the Falls of the Ohio River. Billy Clark accompanied them. At the end of that year Thomas Jefferson, then a delegate from Virginia to the Continental Congress, asked George Rogers Clark if he would lead an expedition to explore the interior of the continent. Clark demurred and a few months later Jefferson went off to be Minister to France. John and Ann Clark meanwhile began settling in to their new home near Louisville. They named their new farm Mulberry Hill. Trouble with Indians continued and in August 1789 William Clark joined the Kentucky militia in a campaign against Wea Indians in southern Indiana. He kept a journal and reported killing four Indian men and four Indian women. In the Spring of 1791 William joined Captain James Brown's company of mounted Kentucky volunteers for a campaign into "Indian country." The overall commander was Brigadier General Charles Scott. During the campaign, Clark participated in a series of engagements under the leadership of then Lieutenant Colonel James Wilkinson that resulted in a large number of Indian deaths. In this campaign and an ensuing one Clark developed a great respect for Wilkinson. In August 1791, Clark was mustered out prior to the November 4 bloody defeat of Brigadier General Arthur St. Clair on the Wabash River. In April 1792, President George Washington appointed Anthony Wayne commander-in-chief of the Continental Army and made Brigadier General Wilkinson his Deputy Commander. Wilkinson recruited Clark as a second lieutenant in Captain John Crawford's rifle company.