On 6 December 1866, at the age of 18, Lady Mordaunt married Sir Charles Mordaunt, 10th baronet, at St. John’s Episcopal Church in Perth.
Sir Charles was a stolid country squire with no interests beyond hunting and shooting, and Harriet was a giddy young beauty. He was a Conservative M.P.
The couple lived at Walton Hall, Warwickshire. The Mordaunts had toured Switzerland together early in their marriage, but when, in June 1868, Sir Charles prepared for an annual fishing trip to Norway, Lady Mordaunt encouraged him to go on his own. Arrangements were made for her to remain at Walton Hall in the company of a sister and another lady. However, when Sir Charles returned early from Norway, he found his wife alone. One of her maids later testified that, during Sir Charles's absence, Lady Mordaunt had been visited in London by Viscount Cole (later 4th Earl of Enniskillen), who, after dinner, had "remained alone with her until a very late hour"; on another occasion, he had travelled with her by train from Paddington station to Reading, where he alighted from a carriage of which they had been the only occupants. Other servants, who seem to have resented Lady Mordaunt's behaviour, added their own accounts of Lord Cole’s visits.
Parliament rose on 28 July 1868, and in November Sir Charles did not stand for re-election. The Liberal Party swept to power at the first general election since Disraeli's Reform Act of 1867. By then, Lady Mordaunt was five months pregnant. On 28 February 1869 she gave birth prematurely to a daughter, Violet Caroline. The timing was significant, in view of Sir Charles's absence on a fishing trip the previous year. Doctors initially feared that the child might be blind, causing Lady Mordaunt to become hysterical, imagining that this had been brought about by a hereditary sexually transmitted disease. (At the time, gossip surrounding Freddy Johnstone, a close friend of the Prince of Wales, whom Lady Mordaunt shortly afterwards claimed to have been one of her lovers, was that he suffered from such a disease.) Violet’s eye infection was successfully treated and no venereal infection was found in either mother or child. However, following this episode, Lady Mordaunt not only declared to her husband, "Charlie, I have deceived you; the child is not yours; it’s Lord Cole’s", but claimed to have committed adultery with "Lord Cole, Sir Frederick Johnstone, the Prince of Wales and others, often and in open day" As one of Princess Alexandra’s biographers put it, "the ensuing scandal was immense".
Sir Charles commenced proceedings for divorce on 20 April 1869.
In view of her nervous and erratic behaviour after Violet’s birth, to which the details were given full rein by her servants, Lady Mordaunt’s family claimed that she was insane and unfit to plead. A counter-affidavit on behalf of Sir Charles maintained that she was feigning a mental disorder. On 30 July 1869, Sir Thomas Moncreiffe, acting as his daughter’s guardian ad litem, formally alleged that, at the time the summons was served on her, she was "not of sound mind". In her diary entry for 6 August, Princess Alexandra noted that "a commission has been ordered to investigate and report if Harriet Mordaunt is truly mad".
The resulting case came up for trial before Lord Penzance in the Court for Divorce and Matrimonial Causes on 23 February 1870. After a trial lasting seven days, and including evidence from the psychiatrist Dr Thomas Harrington Tuke, the jury determined that Lady Mordaunt was suffering from "puerperal mania" at the time the summons was served on her and that she was unable to instruct a lawyer in her defence. Accordingly, Sir Charles's petition for divorce was dismissed, while Lady Mordaunt was committed to an asylum.
Records of the census of 1871 reveal that Lady Mordaunt was living on the western outskirts of London, at the Manor House asylum in Chiswick whose clinical director was Thomas Harrington Tuke. Tuke was, at the time, one of the most respected practitioners in lunacy and, together with the Prince of Wales' physician, William Gull, played a prominent role in the Mordaunt case.
After various legal appeals, including to the House of Lords (in the judicial capacity that it exercised then), Sir Charles Mordaunt’s petition for divorce was remitted to the original court on the basis that Lady Mordaunt’s insanity was not, as a matter of law, a bar to proceedings. Eventually, in 1875, Sir Charles was granted a divorce on the grounds of his wife’s adultery with Lord Cole, who did not contest the action. In 1878 Sir Charles married Mary Louisa Cholmondeley, the daughter of a parson.