After the outbreak of war, the ship was reactivated for active service, initially with III Scouting Group with the High Seas Fleet. "Prinz Heinrich" was used for coastal defense in the North Sea and she participated in the fleet sortie that supported the Raid on Scarborough, Hartlepool and Whitby in December 1914. After the naval command determined that "Prinz Heinrich" was too old to serve in the North Sea against the powerful British Royal Navy, she was transferred to the Baltic Sea in early 1915. She supported offensive minelaying operations and patrolled the central Baltic for Russian forces, but never encountered hostile forces. She provided gunfire support during the attack on Libau in May 1915 and shelled Russian positions during the Battle of the Gulf of Riga in August. In the face of severe crew shortages in late 1915, "Prinz Heinrich" had her crew reduced and was ultimately decommissioned and disarmed in March 1916. She thereafter served in a variety of secondary roles for the rest of the war, before being broken up in 1920.