Earl of Darlington is a title that has been created twice, each time in the Peerage of Great Britain. The first time was in 1722 for the Baroness von Kielmansegg, half-sister1 of King George I. She was created Baroness Brentford at the same time. This creation was for life only, and so the titles expired on her death in 1730.
The second creation came in 1754 in favour of Henry Vane, 3rd Baron Barnard. For more information on this creation, which became extinct in 1891, see the Baron Barnard.
See the Baron Barnard
1 Sophia has often, but erroneously, been asserted to have been mistress of King George I of Great Britain and Ireland: in fact, she was his half-sister. Court ‘observers’ were apparently unable to fathom that George I would have a woman among his intimates without being intimate with her. According to vol. 14 of the Complete Peerage: “The stories that Sophia Charlotte was mistress of George I were demolished in Ragnild Hatton, George I, Elector and King, 1879, pp. 23–4, 134–5.”