During the Siege of Kimberley, Cotty fought as a trooper in the Kimberley Light Horse, a colonial unit of the Boer War of 1899–1902. This is unsurprising as the family were very close to Cecil Rhodes. It was Rhodes who had sponsored the raising of this new regiment, the Kimberley Light Horse, in anticipation of conflict with the Boers and shortly prior to the Siege of Kimberley. The Kimberley regiment is one of only two regiments of the British Empire holding as a Battle Honour the defence of its own city - in this instance Defence of Kimberley. Bill Cotty received the Defence of Kimberley clasp and the Kimberley Star for his service.