The son of the Hon. Spencer Cowper (d1728), Chief Justice of Chester and Attorney General to the Prince of Wales, second son of Sir William Cowper, 2nd Bart of Ratling Court (d 26.11.1706), and of Pennington Goodere, dau of John Goodere.
William Cowper hold the office of Clerk of the Parliaments from 1716-1740 and was succeeded by his younger brother Ashley Cowper (d 1788). He lived at Hertingfordbury Park, co. Hertford which had been bought by his father.He married Joan Budget dau. of John Budget of Chelsea from whom he had two sons and one daughter Mary Cowper (d.1800) who m. to Sir William de Grey, 1st. Lord Walsingham (d.1781). Two of his contemporary famous namesakes were his relatives: William Cowper (c.1665-1773), 1st Earl of Cowper and Lord Chancellor of Great Britain was his uncle and another William Cowper (1731-1800) - the celebrated poet, was his nephew.
His great-great-grandfather Sir William Cowper (1582-1664), created 1st Baronet of Ratling Court, co. Kent in 1642, was a Royalist who loyally supported Charles I and for it he paid with imprisonment by Cromwell and had his estates confiscated, but later reinstated after the Restoration. His son John Copwer, of Lincoln's Inn, also a Royalist died in prison in 1643 during the I Civil War.
Breaking family traditions, the latter's eldest son - Sir William Cowper, 2nd Bart, and his elder son the 1st Earl of Cowper, a distinguished lawyer and brilliant Whig orator in Parliament, made immediate allegiance to William, Prince of Orange on his arrival to England.