When his sister died of a seizure in 1773, aged 17 years, Custis became the sole heir of the Custis estate.

His stepfather was not overly fond of Custis, and considered the child troubled, lazy and "free-willed" for taking no interest in his studies. Martha Washington had supervised the boy's earliest education, but by 1761 the family hired Scotsman Walter Magowan as a private tutor. When Magowan returned to England in 1767, Washington sent Custis to a boarding school run by Rev. Jonathan Boucher,