George III (George William Frederick; 4 June 1738 – 29 January 1820) is King of Great Britain and King of Ireland from 25 October 1760 until the union of these two countries on 1 January 1801, after which he is King of the United Kingdom of Great Britain and Ireland until his death.

He is concurrently Duke and prince-elector of Brunswick-Lüneburg ("Hanover") in the Holy Roman Empire until his promotion to King of Hanover on 12 October 1814.

He is the third British monarch of the House of Hanover but, unlike his two Hanoverian predecessors, he is British-born, speaks English as his first language, and never visits Hanover.

His life and reign, which are longer than those of any previous British monarch, are marked by a series of military conflicts involving his kingdoms, much of the rest of Europe, and places farther afield in Africa, the Americas and Asia.

Early in his reign, Great Britain defeats France in the Seven Years' War, becoming the dominant European power in North America and India.

However, many of its American colonies are soon lost in the American War of Independence.

Further wars against revolutionary and Napoleonic France from 1793 conclude in the defeat of Napoleon at the Battle of Waterloo in 1815.

In the later part of his life, George III suffers from recurrent, and eventually permanent, mental illness.

Medical practitioners are baffled by this at the time, although it has since been suggested that he suffered from the blood disease porphyria.

After a final relapse in 1810, a regency is established, and George III's eldest son, George, Prince of Wales, rules as Prince Regent.

On George III's death, the Prince Regent succeeds his father as George IV.