John Parke Custis (1754–1781), called Jack or Jacky by his relatives and friends, was Martha Washington’s son by her first marriage and the principal heir to the large Custis estate. GW became Jacky’s guardian and the administrator of the Custis estate shortly after he married Martha in January 1759. John Parke Custis received instruction for several years from Anglican clergyman Jonathan Boucher, and later enrolled at King’s College, now Columbia University. Jacky, however, did not pursue his studies, and in February 1774, he married Eleanor (Nelly) Calvert (1754–1811). For two or three years after their marriage, the couple apparently lived alternately at Mount Vernon and Mount Airy, the home of Nelly’s parents in Prince George's County, Maryland. In April 1778, Custis was elected to represent Fairfax County in the Virginia House of Delegates, where he served until his death in 1781. Custis served as a volunteer aide to GW during the siege of Yorktown, where he contracted camp fever and died shortly after the British surrender.

Custis's two eldest children were Elizabeth Parke (Betsy) Custis (1776–1832) and Martha Parke (Patsy) Custis (1777–1854). His two youngest children, Eleanor Parke (Nelly) Custis (1779–1852) and George Washington Parke Custis (1781–1857), made their home with GW and Martha. GW often spoke of the younger grandchildren as “adopted” by him and Martha.