On 5 June 1838, Thellusson married Elizabeth Eleanor, Lady Duff ( Prescott), widow of General Sir James Duff. Elizabeth was a daughter of
Sir George Prescott, 2nd Baronet and Catherine Crichton Mills (a daughter of Sir Thomas Mills,
Governor of Quebec). Together, they had one son and one daughter: • Hon. Anne Blanche Thellusson (1839–1886), who died unmarried. •
Frederick William Brook Thellusson, 5th Baron Rendlesham (1840–1911), who married Lady Egidia Montgomerie, a daughter of
Archibald Montgomerie, 13th Earl of Eglinton. Lady Rendlesham died in December 1840. Lord Rendlesham survived her by twelve years and died in April 1852, aged 54. He was succeeded in the barony by his only son,
Frederick. == References ==