Montagu took his seat in the House of Lords in 1739 and in 1744 was appointed one of the Lords of the Admiralty. He represented the United Kingdom at the negotiations leading to the conclusion of peace in 1748. He then became First Lord of the Admiralty, 1748 to 1761, for a brief period in 1763 and again from 1771 to 1782, after which he held no further public office. A selection of his papers were published by Sir George Barnes and Commander J.H. Owen, 'The private papers of John, Earl of Sandwich 1771-1782' (Navy Records Society, 1932-1938, 4 vols). There is a biography by George Martelli, 'Jemmy Twitcher, a life of the Fourth Earl of Sandwich' (London, 1962).
The papers consist of five volumes of Sandwich's appointment books, 1771 to 1782. These were purchased from the Montagu family in 1957. The remainder of the collection consists of transcripts. In 1956 and 1960 the Secretary of the Navy Records Society deposited on loan transcripts of Sandwich's papers, 1771 to 1782, not included in the Society's publication. The Heritage Lottery Fund has supported the purchase of additions to this collection.
National Maritime Museum, Greenwich, London; Heritage Lottery Fund