Background and education. 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.
Marlborough was born in England on 13 May 1844. He was the eldest son of John Spencer-Churchill, 7th Duke of Marlborough (1822–1883), who served as Lord Lieutenant of Ireland and Lord President of the Council, and Lady Frances Anne Emily Vane (1822–1899). He was the elder brother of Lord Randolph Churchill and the uncle of Winston Churchill.
Comenzó a trabajar en Marlborough: His Life and Times, una biografía en cuatro volúmenes de su antepasado John Churchill, primer duque de Marlborough.   Fue en ese momento que se había ganado la reputación de ser un gran bebedor de bebidas alcohólicas, aunque Jenkins cree que a menudo era exagerado. 
William Cavendish, 4th Duke of Devonshire, KG, PC (8 May 1720 – 2 October 1764), styled Lord Cavendish before 1729, and Marquess of Hartington between 1729 and 1755, was a British Whig statesman and nobleman who was briefly nominal Prime Minister of Great Britain.