After her commissioning, she served with the II Scouting Group, part of the reconnaissance forces of the High Seas Fleet. She was assigned to patrols off the island of Heligoland at the outbreak of World War I in early August 1914. At the Battle of Heligoland Bight on 28 August 1914, the German patrol forces were attacked by superior British forces, including five battlecruisers and several light cruisers. "Mainz" was initially stationed in support of the forces on the patrol line. She attempted to reinforce the beleaguered German forces, and encountered a much stronger force of British cruisers and destroyers. They scored several damaging hits with gunfire and a torpedo that disabled "Mainz" and prompted her commander to abandon ship. The British rescued 348 men from the crew before the ship rolled over and sank. Eighty-nine men were killed in the battle, including her commanding officer.