He became known by the courtesy titleMarquess of Blandford in 1817 when his father succeeded to the dukedom. He sat as a ToryMember 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. 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 the transfer of rotten borough seats to the counties and large towns, the disfranchisement of non-resident voters, the prevention of Crown office-holders from sitting in Parliament, the 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.
As a young man, he and Harriet Caroline Octavia Spencer, daughter of William Robert Spencer, went through a false ceremony of marriage with a relative of the groom posing as a cleric. A voyage to Scotland, where they lived as husband and wife, was intended by the bride and her parents to make this marriage legal under Scottish law. The sixth Duke did, however, successfully contest in a court of law that they had lived as if they had been married. Child by Harriet Caroline Octavia Spencer, who subsequently married Karl Theodor von Westerholt in 1819:
Susan Harriett Elizabeth Churchill, married Aimé Timothée Cuénod.
Lord Alan Spencer-Churchill, married Rosalind Dowker.
After his first wife's death in October 1844, aged 46, he married, secondly, the Hon. Charlotte Augusta Flower, daughter of Henry Flower, 4th Viscount Ashbrook, on 10 June 1846. They had two children:
Lord Almeric Athelstan Spencer-Churchill, died young.
After his second wife's death in April 1850, aged 31, he married, thirdly, his first cousin Jane Francis Clinton Stewart, daughter of the Hon. Edward Richard Stewart and grand-daughter of John Stewart, 7th Earl of Galloway, on 18 October 1851. They had one child:
Lord Edward Spencer-Churchill, married Augusta Warburton, daughter of Major George Drought Warburton, and had issue.
The 6th Duke of Marlborough died at Blenheim Palace on 1 July 1857, aged 63, and was succeeded by his eldest son, John. The Duchess of Marlborough died at 28 Grosvenor Street in Mayfair, London, in March 1897, aged 79.
Literature
Mary Soames; The Profligate Duke: George Spencer Churchill, Fifth Duke of Marlborough, and His Duchess