Harriet Mordaunt (1848–1906), born Harriet Sarah Moncreiffe, was a British aristocrat and the wife of Sir Charles Mordaunt, 10th Baronet. She is primarily known in historical records for her involvement in a high-profile divorce scandal in 1870, which implicated several members of the British upper class and the heir apparent to the British throne.
Born in Scotland to Sir Thomas Moncreiffe and Lady Louisa Hay-Drummond, she married Sir Charles Mordaunt, a Member of Parliament and landowner, in 1866. In 1869, following the birth of a daughter, Lady Mordaunt confessed to her husband that she had engaged in multiple extramarital affairs. Among those she named was Albert Edward, Prince of Wales (the future King Edward VII).
Sir Charles Mordaunt subsequently filed for divorce. The resulting legal proceedings, known as the Mordaunt divorce case, gained significant public attention because the Prince of Wales was summoned to testify as a witness. This marked the first time a Prince of Wales had appeared in court since the 15th century. Although the Prince denied any "improper familiarity" and no evidence was produced to prove adultery on his part, the association caused a considerable scandal for the monarchy.
The initial divorce petition was contested on the grounds of Harriet Mordaunt's mental state. Medical experts testified that she was suffering from "puerperal mania" (a term used at the time for postpartum psychosis) and was unfit to plead. In 1870, the court found her to be legally insane, and the divorce suit was stayed. Lady Mordaunt was subsequently committed to a series of private asylums, including Mowbray House and later Chiswick House.
Sir Charles Mordaunt eventually obtained a divorce in 1875 after presenting evidence of his wife’s adultery with another individual, Viscount Cole. Harriet Mordaunt remained in institutional care for the remainder of her life, dying in May 1906. She was buried at Brompton Cemetery in London.