His Royal Highness General Prince Ernest Augustus William Adolphus George Frederick, KG, Crown Prince of Hanover, Duke of Brunswick-Lüneburg, Prince of the United Kingdom of Great Britain and Ireland, 3rd Duke of Cumberland and Teviotdale, and Earl of Armagh, was born at Hanover during the reign of his grandfather, King Ernst August I. He became the Crown Prince of Hanover upon his father's ascension as King George V in November 1851. King Wilhelm I of Prussia and his minister-president Otto von Bismarck desposed George V for having sided with the defeated Austria in the 1866 Austro-Prussian War. During that war, the Crown Prince saw action at the Battle of Langensalza.
After the war, the exiled Hanoverian royal family took up residence in Hietzing, near Vienna, but spent a good deal of time in Paris. George V never abandoned his claim to the Hanoverian throne and maintained the Guelphic Legion at his own expense. The former Crown Prince travelled during this early period of exile. While visiting his second cousin the Prince of Wales (later King Edward VII) at Sandringham in 1875, he met Princess Thyra of Denmark (29 September 1853-26 February 1933), the youngest daughter of King Christian IX and a sister of the Princess of Wales (later Queen Alexandra).
When King George V died in Paris on 12 June 1878, Prince Ernst August succeeded him as Duke of Cumberland and Teviotdale in the peerage of Great Britain and Earl of Armagh in the peerage of Ireland. Queen Victoria created him a Knight of the Garter on 1 August 1878. On 22 December, he married Princess Thyra at Copenhagen. The Duke and Duchess of Cumberland had six children:
The Duke of Cumberland was partially reconciled with the Hohenzollern dynasty in 1913, when his surviving son, Prince Ernst August, married the only daughter of the German Emperor Wilhelm II, the grandson of the Prussian king who had deposed his father. He renounced his succession rights to the Brunswick duchy (which had belonged to the Guelph-d'Este dynasty or House of Hanover since 1235) on 24 October 1913. In exchange, the younger Ernst August became the reigning Duke of Brunswick on 1 November. Wilhelm II created the elder Ernst August a Knight of the Order of the Black Eagle. The younger Duke Ernst August abdicated his throne along with the other German princes in November 1918.
The outbreak of World War I created a breach between the British Royal Family and its Hanoverian cousins. On 23 March 1919, King George V of Great Britain ordered the removal of the Duke of Cumberland from the Roll of the Order of the Garter. Under the terms of the 1917 Title Deprivation Act, on 28 March 1919 his name was removed from was removed from the roll of Peers of Great Britain and of Ireland by Order of the King in Council for "bearing arms against Great Britain."
Prince Ernst August, the former Crown Prince of Hanover and former Duke of Cumberland, died of a stroke on his estate at Gmunden in November 1923.