On 6 February 1915, the obsolescent light cruiser joined the East African campaign. On 6 July, she engaged the German cruiser and German shore batteries, during the Battle of Rufiji Delta. "Pioneer" remained off East Africa and took part in many bombardments of German East Africa, including Dar-es-Salaam on 13 June 1916. "Pioneer" then returned to Australia, to be decommissioned in October 1916.