On 12 July 1843 at St. George Street, Mayfair, Lady Frances married John Spencer-Churchill, Marquess of Blandford. Upon her marriage she was styled Marchioness of Blandford. The couple made their principal home at the Spencer-Churchill family seat of Blenheim Palace in Woodstock, Oxfordshire. The marriage produced eleven children:
On 1 July 1857, her husband succeeded to the title of 7th Duke of Marlborough, and from that date henceforth, Frances was styled Duchess of Marlborough. She was a commanding and hot-tempered woman described in The Complete Peerage as a "woman of remarkable character and capacity, judicious and tactful". Her face had more strength than beauty and her eyes were either warm or hard, never lacklustre. She ruled Blenheim Palace and its household with an iron hand; yet it was she who rejuvenated the palace with her lavish and gay entertainments which she herself organised; transforming the palace "into a social and political focus for the life of the nation". She was a domineering yet devoted mother; both of her surviving sons' marriages were a disappointment to her. Her eldest son George married a woman described as stupid, pious and dull, while her youngest and favourite son, Lord Randolph earned her displeasure by marrying, against the wishes of both herself and the Duke, American socialite Jennie Jerome, whom Frances openly disliked. Frances and her husband refused to attend Lord Randolph and Jennie's wedding at the British Embassy in Paris, which took place on Frances's 52nd birthday. Like the rest of the 19th-century British aristocracy, the Marlboroughs regarded American women as "strange and abnormal creatures with habits and manners something between a Red Indian and a Gaiety Girl". When the newly-wed couple moved to their home in Curzon Street in London, however, Frances arrived to help Jennie pay her first visits to the leaders of London society. She lent her some of her own jewels for the occasion, and the two women travelled in the Marlborough family coach. Frances featured largely in the lives of the younger members of the family, including her grandson Winston, to whom she often acted as a substitute mother. From 1876 to 1880 her husband served as Lord Lieutenant of Ireland. As the result of her diligent efforts at famine relief in which she displayed humanity, proficiency and leadership that served to avert the effects of the 1879 Irish potato famine, she was invested as a Lady of the Order of Victoria and Albert by Queen Victoria. , the youngest surviving son of Frances and the Duke
Last years
She became a widow in 1883, lost her eldest son, George, in 1892, and on 24 January 1895, her only surviving son, Lord Randolph Churchill, died at her London home in Grosvenor Square. She never stopped mourning Randolph, and harboured much resentment against his wife, whom she had never liked and now criticised for behaviour unbecoming a grieving widow.
Death
Frances died at Blenheim on 16 April 1899, the day after her 77th birthday, having outlived five of her eleven children. She was buried on 21 April 1899 in the family vault beneath Blenheim Chapel. Her grandson Sir Winston Churchill wrote of her: "She was a woman of exceptional capacity, energy and decision".