In the summer of 1940, after Churchill became the prime minister, Nelson moved from Admiralty House to 10 Downing Street, where he had a rivalry with his predecessor: Neville Chamberlain's cat, the Munich Mouser.

[2] Winston Churchill decided to adopt Nelson, a black[3] stray, when he witnessed him chasing off a "huge dog" from Admiralty House, London.

[5] During the Christmas season of 1941 the United States gifted Churchill a large number of items, including catnip for Nelson.

[15][16] On 2 May 1944, at the Commonwealth Prime Ministers' Conference, Nelson was trapped in a fridge whilst attempting to get "an extra helping of milk",[17] but was soon released unharmed.

[18] During meetings with Churchill's war cabinet, Nelson would sit close to Churchill, acting as a "prime ministerial hot water bottle";[2] the prime minister was reported to have mentioned this fact to Rab Butler, stating that through his acts Nelson "save[d] fuel and power", and thereby aided the war effort.