Earl of Oxford and Earl Mortimer was a title in the Peerage of Great Britain. It was created in 1711 for the statesman Robert Harley, with remainder, failing heirs male of his body, to those of his grandfather, Sir Robert Harley. He was made Baron Harley, of Wigmore in the County of Hereford, at the same time, also in the Peerage of Great Britain and with similar remainder as for the earldom. Harley was the eldest son of Sir Edward Harley and the grandson of the aforementioned Sir Robert Harley. The style Earl of Oxford and Earl Mortimer was chosen because the ancient earldom of Oxford, held for many centuries by the de Vere family, had become dormant but not extinct in 1703. Harley claimed the title because of his relationship through marriage to the previous holders, the de Veres. Despite its form (unique in the history of the peerages of the British Isles), it was a single peerage.
Lord Oxford and Mortimer was succeeded by his only son, Edward, the second Earl. Edward's only son, Henry Cavendish Harley, Lord Harley, died as an infant in 1725. He was succeeded according to the special remainder by his first cousin, Edward Harley, the third Earl. He was the son of Edward Harley, brother of the first Earl. He was succeeded by his eldest son, Edward, the fourth Earl. He notably served as a Lord of the Bedchamber and Lord-Lieutenant of Radnorshire. He was childless and was succeeded by his nephew, Edward, the fifth Earl. He was the son of the Right Reverend John Harley, second son of the third Earl. The fifth Earl was succeeded by his second and only surviving son, Alfred, the sixth Earl. He died childless in 1853 when the titles became extinct.
Several other members of the Harley family may also be mentioned. Sir Robert Harley, grandfather of the first Earl, served as Master of the Mint under Charles I. Brilliana, Lady Harley, wife of Sir Robert Harley, was a celebrated letter-writer. Sir Edward Harley, father of the first Earl, represented Herefordshire and Radnor in Parliament. Henrietta Harley, Countess of Oxford and Countess Mortimer, wife of the second Earl, was the daughter and heiress of John Holles, 1st Duke of Newcastle-upon-Tyne. Lady Margaret Harley, daughter and only surviving child of the second Earl, was the wife of William Bentinck, 2nd Duke of Portland and the mother of Prime Minister William Cavendish-Bentinck, 3rd Duke of Portland. Edward Harley, brother of the first Earl and father of the third Earl, sat as Member of Parliament for Leominster and Droitwich. Robert Harley, brother of the third Earl, also represented Leominster in Parliament. The Right Reverend John Harley, second son of the third Earl and father of the fifth Earl, was Bishop of Hereford. The Honourable Thomas Harley, third son of the third Earl, was Lord Mayor of London. Jane Harley, Countess of Oxford and Countess Mortimer, wife of the fifth Earl, was a patron of the Reform movement and a lover of Lord Byron.