When he truned nineteen, William left home to serve in the military. He became a lieutenant in the United States Army. During William's absence, York probably served as a house slave. Later, in 1795, William returned home to help the family run the farm at Mulberry Hill, and York became his personal slave once more. William occasionally traveled on family buisness to New York, Washington, DC, and New Orleans, usually accompanied by York.
In 1800, at the turn of the century, William turned 30. By that time, York was in his late twenties. He had fallen in love with a slave from a nearby farm. Their masters granted them permission to marry.
Marriage between slaves was accomplished by a simple ceremony, usually conducted by the master of the plantation. No public records were kept of the marriages of slaves, so no information about York's wife or whether they had children is known.
In the meantime, William received a letter from Meriwether Lewis, who had become a friend while they were both serving in the army. The letter explained of President Thomas Jefferson's (we just can't get away from Jefferson, can we?) plan to explore the West, all the way to the Pacific Ocean. Lewis asked Clark to help him lead this expedition.
Clark eagerly accepted. He wrote back, "My friend, I join you with hand and Heart."