On the outbreak of the Second World War, the Royal Navy started laying a series of defensive minefields in British coastal waters and the English Channel. From 11 to 16 September 1939, "Adventure", together with the coastal minelayer and the converted train ferries "Shepperton" and "Hampton", escorted by the cruiser and the destroyers of the 19th Destroyer Flotilla, laid 3119 mines in the Straits of Dover, sealing the east end of the Channel from penetration by German submarines. Following the completion of this operation, "Adventure" was deployed in laying mines off the Yorkshire coast.