King George III. Portrait of King George III (1738-1820), king of Great Britain and Ireland (from 1760-1820). George III was a keen scientist and actively supported the endeavour of scientists such as the astronomer William Herschel and the horologist John Harrison. George III is probably best known as the monarch who lost the colonies to the American revolution in 1783. He is also well known for his mental illness that became so bad that his son, later George IV, took over as Prince Regent in 1811. It is thought that George III's dementia was caused by porphyria, a metabolic disorder.