285
Number of enslaved Africans listed in the October 1759 estate inventory of Daniel Parke Custis (Martha Washington's first husband). Custis dies in 1757 without a will, so the widow is granted a dower share — the lifetime use of 1/3 of the estate's assets. Her dower share, along with the rest of her late husband's estate (including the enslaved Africans), is held in trust for their son Jacky (born 1754).
At least 85
Number of enslaved Africans assigned to the widow Martha Custis as part of the dower share of her late husband's estate. Because she does not own, but has the lifetime use of these enslaved Africans and of their increase (future children and grandchildren), they are called dower slaves. [NOTE: The exact number is unclear because the inventory does not list all children individually.] The 200 or so additional Custis estate slaves (and their increase) continue to farm the Custis plantations but George and Martha Washington receive no economic benefit from their work; rather it all accrues to the benefit of Martha's son Jacky.
1759
Year in which the widow Martha Custis marries Colonel George Washington, on January 9.
~36
Estimated number of enslaved Africans owned by George Washington at the time of his marriage. Using his new wife's wealth, he buys land, more than doubling the size of Mount Vernon. Most of this land is farmed by his wife's dower slaves, but Washington also buys more enslaved Africans himself. In 1760 he pays taxes on 49 enslaved Africans; in 1770 on 87 enslaved Africans; and in 1774 on 135 enslaved Africans. [NOTE: These numbers do not include the dower slaves.] Washington's last recorded purchase of enslaved Africans is in 1772, but he later receives a few others in repayment of debts.
1775
Year in which Jacky Custis turns 21, inheriting two-thirds of his father's estate (his mother's dower share is held in trust for him until her death). Jacky dies in 1781, leaving a widow and four children. His estate, plus the 1/3 of his father's estate controlled by his mother (the dower share), is held in trust for his children.
113
Number of dower slaves listed in the 1786 Mount Vernon slave census. The increase is due to the dower mothers having children. All children of dower mothers are themselves dower slaves.
103
Number of "Washington" slaves listed in the 1786 Mount Vernon slave census.
1790 Census
3,893,635
Total population of the United States in 1790 according to the U.S. Census.
694,280 / 59,150
Population of enslaved Africans / free blacks in the United States in 1790.
President Washington in New York City (1789-90)
340,120
Total population of New York (State) in 1790 according to the U.S. Census.
21,324 / 4,682
Population of enslaved Africans / free blacks in New York (State) in 1790.
7
Number of enslaved Africans that Washington brings to New York City in 1789 to work in the presidential household: Will Lee, Moll, Austin, Oney Judge, Giles, Paris and Christopher Sheels.
President Washington in Philadelphia (1790-97)
434,373
Total population of Pennsylvania in 1790 according to the U.S. Census.
3,737 / 6,537
Population of enslaved Africans / free blacks in Pennsylvania in 1790.
8
Number of enslaved Africans that Washington brings to Philadelphia in November 1790 to work in the President's House: Moll, Austin, Oney Judge, Giles, Paris, Christopher Sheels, Hercules and Richmond.
1
Number of enslaved Africans that Washington subsequently brings to Philadelphia. "Postilion Joe" first appears in the President's House documentary record in 1795.
2
Number of President's House enslaved Africans who successfully escape to freedom from Philadelphia: Oney Judge and Hercules.
1791
Year in which an amendment is proposed (and fails) in the Pennsylvania Assembly to exempt all slave-holding officers of the federal government (including Washington, his Cabinet, and the Supreme Court) from the Gradual Abolition Act. This was an attempt to make the state more hospitable to slave-holders in hopes of having Philadelphia become the permanent capital of the United States. The proposal is withdrawn before debate after heated opposition from the Pennsylvania Abolition Society.
1793
Year in which the U.S. Congress passes and Washington signs the Fugitive Slave Act. The U.S. Constitution (Article IV, Section 2) guaranteed the right of a slave-holder to recover a runaway slave. The Fugitive Slave Act establishes the legal mechanism for accomplishing this, makes it a federal crime to assist an escaping slave or interfere with his recapture, and sets severe fines for doing so. The Fugitive Slave Act allows slave-catchers into every U.S. state and territory.
47 to 8
Margin by which the U.S. House of Representatives passes the Fugitive Slave Act. The U.S. Senate also passes the Act, but the vote count is not recorded. Washington makes no known comment on the Act, and signs it into law on February 12, 1793 (probably in his private office in the President's House).
1/5
Fraction of the American population that is of African descent, all of whom are affected by the 1793 Fugitive Slave Act. There is no safe haven for an escaped slave anywhere in the U.S. because of this law, and even free blacks are in danger of being kidnapped and sold into slavery by unscrupulous slave-catchers.
President Washington in Retirement (1797-99)
2
Number of President's House enslaved Africans who return to Mount Vernon with Washington at the end of his presidency: Moll and "Postilion Joe." Christopher Sheels, Richmond, Giles and Paris were returned to Mount Vernon in 1791. Austin died in 1794 enroute from Philadelphia to Mount Vernon. Oney Judge escaped to freedom in May or June 1796 from the President's House, and Hercules escaped in March 1797, reportedly on the night before Washington leaves Philadelphia. [What became of the 9 enslaved Africans? click to see chart]
At least 2
Number of former President's House enslaved Africans who attempt to escape to freedom from Mount Vernon, but are unsuccessful. There are presumed escape attempts by Richmond in November 1796, and by Christopher Sheels in September 1799.
1798
Year in which Washington's nephew, Burnwell Bassett Jr., travels to New Hampshire in an attempt to recapture Oney Judge. Oney is now married to a freeman, Jack Staines, but legally she and their infant daughter are dower slaves (because Oney is enslaved, her marriage is not legally recognized and Jack Staines has no legal relationship to his own child). Oney goes into hiding, foiling Bassett's plan to abduct her. She later has another daughter and a son with Staines, but he and all three children predecease her.
153
Number of dower slaves listed in the 1799 Mount Vernon slave census.
124
Number of "Washington" slaves listed in the 1799 Mount Vernon slave census.
1799
Year in which Washington dies, on December 14. In his will, Washington designates that his enslaved Africans be freed upon his wife's death.
1801
Year in which Washington's enslaved Africans are freed, on January 1. Martha Washington decides not to wait until her death to free her late husband's slaves.
At least 12
Number of marriages between dower and "Washington" slaves. Legal status is traced through the female, so the children of a "Washington" father and a dower mother (such as Hercules and his late wife Alice) are themselves dower slaves, cannot be freed by Washington's will, and remain enslaved for life. Children of a dower father and a "Washington" mother (such as "Postilion Joe" and his wife Sall) are freed by Washington's will. Joe remains enslaved, but Sall and their children are freed, and take the last name Richardson.
1802
Year in which Martha Washington dies, on May 22. In her will, she bequeaths to her grandson George Washington Parke Custis the one enslaved African she owns outright: Elisha. The dower slaves (who had numbered 153 people in 1799) are divided among her four grandchildren (the children of Jacky Custis). Jacky Custis's own enslaved Africans (who had numbered 272 people soon after his 1781 death) are distributed as each of his heirs reaches majority.
1848
Year in which Oney Judge dies, on February 25, in Greenland, New Hampshire. Because of the 1793 Fugitive Slave Act, passed overwhelmingly by Congress and signed into law by Washington at the President's House, Oney Judge spends the last 52 years of her life as a fugitive.

Information courtesy of ushistory.org