Sir Winston Churchill's grandfather: The Seventh Duke of Marlborough. Sir Winston Churchill's great descent is illustrated on other pages by a family tree and family portraits. His greatest ancestor was the first Duke of Marlborough, victor of Blenheim, Malplaquet and Oudenarde, but his more immediate forbears were also of distinction. His grandfather, the seventh Duke of Marlborough KG sat as Conservative Member for Woodstock 1844-1845 and from 1847 to 1857, and was Lord Lieutenant of Oxfordshire, Lord President of the Council, 1868, and Viceroy of Ireland 1876-1880.