The Hamilton–Mohun Duel occurred on 15 November 1712 in Hyde Park, then on the outskirts of London. The principal participants were James Hamilton, 4th Duke of Hamilton, and Charles Mohun, 4th Baron Mohun. Both men died from wounds received.^[1]

Although ostensibly fought over a disputed inheritance, the duel had strong political overtones. Mohun was a prominent Whig while Hamilton had close links to the Tory government of Robert Harley.^[2]: 18 Hamilton had recently been appointed as British Ambassador to Paris where he was expected to negotiate the peace agreement that would end the War of the Spanish Succession. Mohun's political patron the Duke of Marlborough had recently been dismissed from his command, and was strongly opposed to the peace plan. This may have motivated Mohun to issue his challenge.^[2]: 18

Mohun had developed a reputation as a violent and frequent duellist, having been involved in several fatal encounters. His father, the 3rd Baron Mohun, had himself been killed in a duel.^{[2]: 17–18}

Hamilton accepted Mohun's challenge to fight with swords. Hamilton selected his relation Colonel John Hamilton as his second, while Mohun employed his friend and political ally, Irish officer George Macartney.^[2]: 18 As was often customary at the time, seconds actively engaged in the combat.^[3]: 79

The group assembled in Hyde Park very early in the morning. Once the duel began, Hamilton and Mohun went at each other "like wild beasts, not fencing or parrying".^[4]: 19 In these fierce exchanges both Mohun and Hamilton were wounded. Mohun was run through the chest, a fatal wound, while Hamilton was cut on the arm. Exactly what happened next remained contentious. Colonel Hamilton claimed that, with the duel over, he went to the assistance of the Duke who had dropped his sword. Macartney stepped forward and delivered a fatal blow to him. The Duke soon died of his wounds.^[1]

The two seconds, Macartney and Colonel Hamilton were both charged as accessories to murder. While Hamilton gave himself up, Macartney fled into exile in Hanover. He only returned to Britain once George I came to the throne.^[4]: 20 Based on Hamilton's testimony, Tories in Parliament portrayed the whole affair as a Whig plot designed to derail the prospective peace agreement with France.^[2]: 18

After being put on trial in December 1712, Colonel Hamilton was found guilty of manslaughter. He received a much lesser punishment than he might potentially have been given because the jury accepted his claim that he had not known a duel was to take place when he arrived at the park.^[2]: 19

The apparent savagery of the duel led to a public outcry. This reinvigorated the campaign to clamp down on dueling, and, in April 1713, Queen Anne spoke out against the practice.^[2]: 20 While dueling continued to be a popular way of settling disputes during the eighteenth century, fresh conventions developed such as the use of pistols rather than swords. The traditional involvement of seconds in the actual fighting rapidly declined.^[2]: 116

The duel forms the basis for a scene in William Makepeace Thackeray's 1852 novel The History of Henry Esmond.