King was born in Edinburgh and was the daughter of Admiral W. A. Baillie Hamilton and Lady Harriet Hamilton, sister of the Duke of Abercorn. In 1864, she married the banker and publisher, Henry Samuel King. She lived at the Manor House, Chigwell, Essex, all her married life, but after her husband's death in 1878 she moved with her children to another part of the country. Her strong sympathy for Mazzini and the cause of Italian unification inspired a number of her works. She was received into the Roman Catholic Church in 1890 by Cardinal Manning.