George III ( Born George William Frederick 4 June 1738 - 29 January 1820 ) was King of Great Britain and King of Ireland from 25 October 1760 until 1 January 1801 . He was then King of the United Kingdom of Great Britain and Ireland until his death . George III ( George William Frederick ; 4 June 1738 â `` 29 January 1820 ( N.S. ) ) was 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 was King of the United Kingdom of Great Britain and Ireland until his death .