His first confirmed episode of what contemporaries called madness at the time was in 1788, 28 years into his reign. This is the event portrayed in Alan Bennett’s play The Madness of George III and the film based upon it, The Madness of King George.
There is an amusing story behind the title change from the play to the film. Bennett relates it in his introduction to the film’s screenplay. American investors in the film project objected to the play’s title. Why? Because Americans would look at it and think, “I didn’t see the Madness of King George parts one or two, so why should I go see part three?”
Those who have seen the film or play know that George recovered his wits after a few months, a period known as the Regency Crisis. The crisis threatened to bring down the Tory government of William Pitt the Younger. The reason is that many MPs, mostly opposition Whigs, were demanding that the king’s eldest son George, Prince of Wales, be installed as regent to rule in his father’s name. The king himself furiously opposed the idea.
The Prince at the time favoured the Whigs, who believed he would help them gain power. The king’s recovery ended that prospect and the Tories remained in power for most of the next thirty-four years. (Image: George, Prince of Wales, later George IV, c. 1789, by Mather Byles Brown)
On this occasion, the disease became permanent and progressed to dementia. In addition, the king was now blind from cataracts, losing his hearing, and in great pain from rheumatism. He had become Percy Shelley’s “old, mad, blind, and dying king.” (Image: George III, by Henry Meyer, a sketch done during the king's last years)
Despite his various health problems, George III clung to life for another ten years, dying at age 81 in 1820. Upon his death, The Prince Regent became king as George IV (1820-1830). His twenty years as Regent and King are still referred to as the Regency Period.
Biographers and Historians of Psychiatry have long debated the nature of George III’s illness. In the late 1960s, psychiatrists Ida Macalpine and Richard Hunter argued that George suffered from a genetic metabolic disorder called porphyria, which among other things, can cause one’s urine to turn dark red or purple. Bennet’s play and film emphasizes the king’s dark urine and the bumbling doctors who think it unimportant.
Other researchers have questioned the porphyria diagnosis. They argue that the king showed symptoms of psychoses such as dementia, mania, and manic-depressive or bipolar disorder. A study of his hair in 2005 revealed that he had consumed large amounts of medicines or cosmetics containing arsenic, a poison that might have precipitated his disease.
Interestingly, George III was attacked on several occasions by people later declared insane. In 1786, a woman named Margaret Nicholson tried to stab him with a small dessert knife. The king easily fended of the blow and told his attendants to treat her kindly. “The poor creature is mad. Do not hurt her. She has not hurt me.” (Image: Contemporary print showing Nicholson's attack on the king)
In 1790 he reacted with similar sympathy when John Frith, who believed he was St. Paul, threw a rock at the king’s coach. A third assailant, James Hadfield, tried and failed to shoot the king in 1800 at the Theatre Royal, Drury Lane. The king seems to have been unperturbed by the incident. He fell asleep during the interval.
All three assailants were sent to Bethlem Hospital for the Insane, Hadfield after being declared not guilty due to insanity during his precedent-setting trial for treason. (Image: Contemporary print showing Hadfield's attempt to shoot the king).
Ida Macalpine and Richard Hunter, George III and the Mad Business (London: Allen Lane, 1969)
Victoria Howard, “George III and His Madness,” Crown Chronicles, “9 April, 2015 https://thecrownchronicles.co.uk/history/history-posts/george-iii-and-his-madness/
"What was the truth about the madness of George III?” https://www.bbc.co.uk/news/magazine-22122407