On the morning of 28 August, "Mainz" was at anchor in the mouth of the Ems; her sister , the flagship of "Konteradmiral" (Rear Admiral) Leberecht Maass was re-coaling in Wilhelmshaven, lay in the entrance to the Weser. These three cruisers were assigned to support the cruisers and , and the aviso , which were stationed on the patrol line that morning. At 07:57, the Harwich Force encountered the outer German torpedo boats, which fled back to the German cruisers. In the ensuing Battle of Heligoland Bight, "Stettin" engaged the British force first, and was quickly reinforced by "Frauenlob". At 09:47, "Mainz" was ordered to steam out behind the British to cut off their line of retreat. She got under way by 10:00, and operated in conjunction with a floatplane used for reconnaissance.