George III was King of Great Britain and Ireland from 25 October 1760 until his death in 1820. The Acts of Union 1800 unified Great Britain and Ireland into the United Kingdom of Great Britain and Ireland, with George as its king. He was concurrently Duke and Prince-elector of Hanover in the Holy Roman Empire before becoming King of Hanover on 12 October 1814. He was a monarch of the House of Hanover, who, unlike his two predecessors, was born in Great Britain, spoke English as his first language, and never visited Hanover.

The Kingdom of Great Britain was a sovereign state in Western Europe from 1707 to the end of 1800. The state was created by the 1706 Treaty of Union and ratified by the Acts of Union 1707, which united the kingdoms of England and Scotland to form a single kingdom encompassing the whole island of Great Britain and its outlying islands, with the exception of the Isle of Man and the Channel Islands. The unitary state was governed by a single parliament at the Palace of Westminster, but distinct legal systems—English law and Scots law—remained in use.