Sir Hugh's illegitimate son James Smithson, otherwise Jacques Louis Macie, born in about 1764 to one of Elizabeth's cousins, bequeathed the fortune which established the Smithsonian Institution.

She became a duchess in 1766 when her husband was created Duke of Northumberland, and on her death in 1776, aged sixty, her barony and the earldom of Northumberland passed to her eldest son, Hugh, who inherited his father's dukedom ten years later.

Their children were: Elizabeth Percy is buried in the Northumberland Vault, within Westminster Abbey.

Her epitaph describes her as having "every amiable & benevolent virtue", and as "an ornament of courts, an honour to her country, & patern to the great, a protectress of the poor, ever distinguished for the most tender affection for her family & friends".

[3] In 1775, her diary of her travels in the Dutch Republic, called "A Short Tour made in the Year One Thousand Seven Hundred and Seventy One" was published, although anonymously.