Cowper had already been created Baron Cowper of Wingham in the County of Kent, in the Peerage of England on 14 December 1706, with normal remainder to the heirs male of his body, and was made Viscount Fordwich, in the County of Kent, at the same time as he was given the earldom, also Peerage of Great Britain and with similar remainder.
He was the great-grandson of William Cowper, who was created a Baronet, of Ratling Court in the County of Kent, in the Baronetage of England on 4 March 1642.
Cowper married Lady Henrietta, younger daughter of Henry de Nassau d'Auverquerque, 1st Earl of Grantham, a relative of the Dutch House of Orange-Nassau, and a count of the Holy Roman Empire.
On the death of Lady Cowper's elder sister, Lady Frances Elliot, in 1772, the second Earl's son, the third Earl became Lord Grantham's heir general, and on 31 January 1778 he was created a Prince of the Holy Roman Empire (Reichsfürst) by the Emperor Joseph II.
Lord Cowper's second son, the fifth Earl (who succeeded on the early death of his unmarried elder brother), was a Fellow of the Royal Society.
Spencer Cowper, younger son of the second Baronet and brother of the first Earl, was a politician and barrister.