"Mecklenburg" and her sisters were not included in the German fleet that assaulted the Gulf of Riga in August 1915, due to the scarcity of escorts. The increasingly active British submarines forced the Germans to employ more destroyers to protect the capital ships. "Mecklenburg" took part in two sweeps to Huvudskär on 9–11 and 21–23 September. On 17 December she ran aground in the entrance to the harbor of Libau, but was towed free without suffering any damage. She was to replace the worn-out armored cruiser in the reconnaissance forces of the fleet in the Baltic, but "Mecklenburg" and her sisters were removed from service shortly thereafter. By this stage of the war, the German Navy was facing severe shortages of crews, which could be alleviated by the decommissioning of older, less effective warships. Furthermore, the increasing threat from British submarines and Russian mines in the Baltic by 1916, the latter of which sank the armored cruiser , convinced the German navy to withdraw the elderly "Wittelsbach"-class ships from active service. On 6 January 1916, "Mecklenburg" left Libau bound for Kiel, arriving the following day. She was decommissioned on 24 January and placed in reserve.