He succeeded to the title in 1799, and in 1805 married Emily Mary Lamb, daughter of Sir Peniston Lamb, first Viscount Melbourne.

CORRESPONDENCE OF Peter Leopold Louis Francis Nassau Cowper, fifth earl Cowper:

Lord Cowper to Lady Melbourne, [May] 1805 in In Whig Society 1775-1818: Compiled from the hitherto unpublished Correspondence of Elizabeth, Viscountess Melbourne, and Emily Lamb, Countess Cowper, afterwards Viscountess Palmerston (London: Hodder and Stoughton LTD., 1921)

Lord Cowper to Emily Lamb, [May 1805] in In Whig Society 1775-1818: Compiled from the hitherto unpublished Correspondence of Elizabeth, Viscountess Melbourne, and Emily Lamb, Countess Cowper, afterwards Viscountess Palmerston (London: Hodder and Stoughton LTD., 1921)