In late 1810, at the height of his popularity,[126] King George, already virtually blind with cataracts and in pain from rheumatism, suffered a relapse into his mental disorder and became dangerously ill. In his view the malady had been triggered by stress over the death of his youngest and favourite daughter, Princess Amelia.[127] The princess's nurse reported that "the scenes of distress and crying every day ... were melancholy beyond description."[128] George accepted the need for the Regency Act 1811,[129] and the Prince of Wales (later George IV) acted as regent for the remainder of the king's life. Despite signs of a recovery in May 1811, by the end of the year George III had become permanently insane, and lived in seclusion at Windsor Castle until his death.[130]
Prime Minister Spencer Perceval was assassinated in 1812 and was replaced by Lord Liverpool. Liverpool oversaw British victory in the Napoleonic Wars. The subsequent Congress of Vienna led to significant territorial gains for Hanover, which was elevated from an electorate to a kingdom.
Meanwhile, George's health deteriorated. He developed dementia, and became completely blind and increasingly deaf. He was incapable of knowing or understanding that he was declared King of Hanover in 1814, or that his wife died in 1818.[131] At Christmas 1819, he spoke nonsense for 58 hours, and for the last few weeks of his life was unable to walk.[132] He died, of pneumonia, at Windsor Castle at 8:38 pm on 29 January 1820, six days after the death of his fourth son Prince Edward, Duke of Kent and Strathearn.[133] His favourite son, Prince Frederick, Duke of York and Albany, was with him.[134] George III lay in state for two days, and his funeral and interment took place on 16 February in St George's Chapel, Windsor Castle.[133][135][136]