After the outbreak of World War I at the beginning of August 1914, the cruiser was the only vessel on station to guard the German Bight. "Mainz" and IV Torpedo-boat Flotilla were immediately dispatched on 3 August to reinforce the defenses. Other cruisers were also sent to strengthen the forces that were tasked with patrol duties in the southern portion of the German Bight, the Heligoland Bight. The cruisers were divided with the torpedo boat flotillas and were assigned to rotate through nightly patrols into the North Sea. As part of this operation, "Mainz" conducted a patrol on the night of 16 August with the VIII Torpedo-boat Flotilla, without incident. At the same time, British submarines began reconnoitering the German patrol lines. On the night of 21–22 August, "Mainz" provided distant support to a patrol of torpedo boats that inspected fishing vessels in the Dogger Bank. Another foray into the North Sea followed on 23 August; "Mainz" and the torpedo boats escorted a group of minelayers that laid a series of minefields off the mouths of the River Tyne and the Humber before returning to port on 26 August.