British Army Officer. Born into a Loyalist family during the American Revolution, after the war young William and his family departed the United States for England where they settled in Yorkshire and later in London. He entered the British Army in 1792 as a cornet in the 16th Light Dragoons. He was promoted to lieutenant in 1793, and transferred to the 80th Regiment of Foot. In 1794, he purchased a captaincy. He transferred back to the cavalry in 1796, and was assigned to the 17th Light Dragoons. In 1799, he was promoted to major and posted to the 45th Regiment of Foot. He served throughout the Napoleonic Wars, including service in the Peninsular campaign where he was present at the battles of Oporto (1809), Ciudad Rodrigo (1811), and the Battle of Vittoria (1813). For his services, he was invested a Knight Commander of the Order of the Bath (KCB). When Napoleon escaped from his exile on Elba in 1815, Field Marshal Sir Arthur Wellesley, Duke of Wellington appointed DeLancey the Deputy Quartermaster-General of the British Army stationed in Belgium. At the Battle of Waterloo (June 18, 1815), he served as Wellington's aide-de-camp (ADC), and was speaking with the Duke when a ricocheting cannonball struck him from behind. The force of the cannonball knocked him off his horse, breaking eight ribs and causing internal injuries. His wife, Lady Magdalene (Dunglass) DeLancey, stayed by his side throughout the agony of his mortal wounds until his death a week after the battle; they had been married only ten weeks. The Duke of Wellington vividly recalled the wounding and death of his friend, and his account was later published as were Lady DeLancey's memories of the battle and her husband's tragic death. In the film "Waterloo" (released in 1970), DeLancey was portrayed by Ian Ogilvy.