George Spencer-Churchill, 6th Duke of Marlborough (27 December 1793 – 1 July 1857), styled Earl of Sunderland until 1817 and Marquess of Blandford between 1817 and 1840, was a British nobleman, politician, and peer. The great-grandfather of Sir Winston Churchill, he served as Lord-Lieutenant of Oxfordshire between 1842 and 1857.
Styled Earl of Sunderland from birth, he was born at Bill Hill, Hurst, Berkshire (an estate his father was renting at the time), the eldest son of George Spencer, Marquess of Blandford (later George Spencer-Churchill, 5th Duke of Marlborough) and his wife, the former Lady Susan Stewart, daughter of John Stewart, 7th Earl of Galloway. He was educated at Eton between 1805 and 1811, and later at Christ Church, Oxford. He was also given an honorary Doctorate of Civil Laws by Oxford University on 15 June 1841.
He became known by the courtesy title Marquess of Blandford in 1817, when his father succeeded to the dukedom. He sat as a Tory Member of Parliament for Chippenham between 1818 and 1820, and for Woodstock from 1826 to 1831, from 1832 to 1835 and from 1838 to 1840, when he succeeded to the dukedom and entered the House of Lords. In 1842, he was appointed Lord Lieutenant of Oxfordshire, a post he held until his death. On 20 March 1845 he was appointed Lieutenant-Colonel Commandant of the Queen's Own Oxfordshire Yeomanry in which his two eldest sons also served.
In parliament, Blandford became an Ultra-Tory, splitting with Wellington in opposition to Catholic emancipation. In response to the Roman Catholic Relief Act 1829, Blandford introduced the first major reform bill in February 1830, calling for transfer of rotten borough seats to the counties and large towns, disfranchisement of non-resident voters, prevention of holders of office under the Crown from sitting in Parliament, payment of a salary to MPs, and the general franchise for men who owned property. He believed that somewhat more open elections could be relied upon to oppose Catholicism.