The Earl succeeded his father in 1857. A Knight of the Order of the Garter and Privy Counsellor, Spencer served as Lord Lieutenant and custos rotulorum for Northamptonshire from 1872-1908. He was an honorary Colonel in the Northamptonshire Yeomanry, and an Elder Brother of Trinity House. He held and M.A. degree from Cambridge, an honorary Doctorate of Laws from Cambridge (Dublin and Wales), and an honorary Doctorate of Civil Law from Oxford. He also served as Chancellor of the Victoria University. In 1857 he was a Member of Parliament for South Northamptonshire. He then became a Groom of the Stole to the Prince Consort (1859-61) and to King Edward VII (when Prince of Wales 1862-6). During the years 1868-74 and 1882-5 he was Lord Lieutenant of Ireland, then Lord President of the Council (1880-3 and 1886); First Lord of the Admiralty (1892-5); member of the Prince of Wales' Council (1898-1901); Keeper of the Privy Seal and member of the Council of the Duchy of Cornwall (1901-7). In 1858 he married Charlotte Frances Frederica, a Lady of the Royal Order of Victoria and Albert [d. 1903], and daughter of Frederick Charles William Seymour. They had no children; thus John Poyntz Spencer was succeeded in the Earldom by his half-brother, Charles Robert.
Cooper, Douglas, ed. Great Family Collections. New York, 1965:197-240