His Royal Highness Field Marshal The Prince Ernest Augustus, KG, KP, GCB, GCH, Duke of Cumberland and Teviotdale and Earl of Armagh, later His Majesty Ernst August I, King of Hanover and Duke of Brunswick-Lüneburg, was born at Buckingham Palace. He received tutoring at home before proceeding to the University of Göttingen in Germany in summer 1786 along with his younger brothers, Prince Adolphus and Prince Augustus. In 1791, he and Prince Adolphus went to Hanover to receive military training under the supervision of Field Marshal von Freytag. He learned cavalry drill and tactics under Captain von Linsinger of the Queen's Light Dargoons. He proved to be an excellent horseman and good shot, despite his short-sightedness. After only two months of training, von Freytag was so impressed by the prince's progress that he gave him a place with the cavalry as captain.
In March 1792, the Army officially commissioned Prince Ernest Augustus with the rank of colonel in the 9th Hanoverian Light Dragoons. The following year, he gained the command of the 1st Brigade of Cavalry. He spent the War of the First Coalition (1793 - 1797) stationed in Tourany, the headquarters of his elder brother the Duke of York, then commander of the combined British, Hanoverian, and Austrian forces. He lost his left eye during the battle of Tourany (Battle of Cayghem) in early 1794.
Prince Ernest returned to Britain for the first time since 1786 to convalesce. He returned to the continent the following year, and commanded the rear guard of the British army's retreat through Holland. He gained promotion to lieutenant general in 1798 and to general in 1803. On 29 March 1813, he became a field marshal. He served as honorary colonel of the 15th (The King's) Regiment of (Light) Dragoons (Hussars) from 1801 to 1827 and as colonel of the Royal Horse Guards from 1827 to 1830.
On 29 August 1799, George III created Prince Ernest Augustus Duke of Cumberland and Teviotdale and Earl of Armagh. On 29 May 1815, the Duke of Cumberland married his first cousin, Frederica (2 March 1778 - 29 June 1841), the daughter of Karl V, Grand Duke of Mecklenburg-Strelitz. She was also the former wife of Prince Ludwig of Prussia and the widow of Friedrich Wilhelm, Prince of Solms-Braunfels. Queen Charlotte opposed the marriage, even though her future daughter-in-law was also her niece. From her first two marriages, the new Duchess of Cumberland had eight children; from her marriage to Ernest, she had a further three children, only one of whom survived -- a son, who would become George V of Hanover.
The Duke of Cumberland became a Knight of the Garter in 1786. His elder brother, the Prince Regent (later King George IV), created him a Knight Grand Cross of the Order of the Bath in 1815. In 1831, the Duke of Cumberland became a Knight of St. Patrick. Finally, upon his ascension to the Hanoverian throne in 1837, he became Sovereign and Grand Master of the Royal Guelphic Order.
The Duke of Cumberland had a reputation as one of the least pleasant of the sons of George III. Politically an arch-reactionary, he opposed the 1828 Catholic Emancipation Bill proposed by the government of the Prime Minister, the Duke of Wellington. Rumor strongly suggested that he had murdered his valet, and other horrific stories told about him included rumors of incestuous relations with his sister Sophia.
On 20 June 1837, King William IV died, and his niece, Victoria, the only child of the late Duke of Kent, the third son of George III, succeeded as queen of the United Kingdom. However, Salic Law still prevailed in Hanover, meaning that William's legitimate male heir, the Duke of Cumberland, became King of Hanover and Duke of Brunswick-Lüneburg. Hanover and Great Britain thereby gained diverging royal houses.
King Ernst August proved an unpopular ruler, repealing the relatively liberal constitution promoted by his predecessors and ruling as an absolute monarch until forced to grant a constitution by the revolutions of 1848. Both he and Queen Frederica lie buried at Schloss Herrenhausen.