On the outbreak of World War I Laconia was converted into an armed merchant cruiser in 1914 and based at Simon's Town, South Africa in the South Atlantic, from which she patrolled the South Atlantic and Indian Ocean until April 1915. She was then used as a headquarters ship for the operations to capture Tanga and the colony of German East Africa. Four months later she returned to the patrolling of the South Atlantic. She was handed back to Cunard in July 1916 and on 9 September resumed service.