George Washington is considered one of the only politically prominent members of the Virginia aristocracy to free his slaves at his death. According to Joseph Ellis in His Excellency: George Washington, he had been struggling with the issue for 30 years and had been trying to figure out how to do it for the five years before he drew up his will. In his “Schedule of Property” in his will, he listed 317 slaves at Mt. Vernon. He owned 124 and leased 40 more. The rest were not his.
In his will, he wrote: “Upon the decease of my wife, it is my Will & desire that all the slaves which I hold in my own right, shall receive their freedom.”
His most famous slave was his valet, William "Billy" Lee, who was with him most of his life. In the will, he provided Lee with his freedom outright and gave him a small annuity for room and board.
For more on the history of slavery in the U.S., check out:
The Birth of Black America: The First African-Americans and the Pursuit of Freedom at Jamestown by Tim Hashaw
Inhuman Bondage: The Rise and Fall of Slavery in the New World
by David Davis
The Forgotten Fifth: African-Americans in the Age of Revolution
by Gary Nash
Slavery and the Making of America by James Horton
In Hope of Liberty: Culture, Community and Protest Among Northern Free Blacks by James Horton