Who was the first monarch to reign over the United Kingdom of Great Britain and Ireland when they were established as a sovereign state?

The United Kingdom of Great Britain and Ireland was established as a sovereign state on 1 January 1801 by the Acts of Union 1800, which merged the kingdoms of Great Britain and Ireland.

George III (George William Frederick; 4 June 1738 – 29 January 1820) was King of Great Britain and King of Ireland from 25 October 1760 until the union of the two countries on 1 January 1801, after which he was King of the United Kingdom of Great Britain and Ireland until his death. He was concurrently Duke and prince-elector of Brunswick-Lüneburg ("Hanover") in the Holy Roman Empire before becoming King of Hanover on 12 October 1814. He was the third British monarch of the House of Hanover, but unlike his two predecessors, he was born in Britain, spoke English as his first language, and never visited Hanover.

George III lived for 81 years and 239 days and reigned for 59 years and 96 days: both his life and his reign were longer than those of any of his predecessors. Only Victoria and Elizabeth II have since lived and reigned longer.