Prinz Heinrich underwent modernization and conversion into a dedicated training ship in 1914, and the work was completed just before the outbreak of World War I in July that year.

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.

[1] In fact, the armor layout pioneered in Prinz Heinrich provided the basis of all German capital ships designed over the next forty years, including the final battleships of the Bismarck and H-classes.

[5] Nevertheless, she was not without critics; Vizeadmiral (VAdm—Vice Admiral) Albert Hopman referred to the ship as "cheap, but bad" in his memoirs Logbuch eines Seeoffiziers (Log of a Sea Officer).

The naval historians Hans Hildebrand, Albert Röhr, and Hans-Otto Steinmetz regard Hopman's criticism as an exaggeration, pointing out that Prinz Heinrich compared well to foreign contemporaries like the French Desaix, the Russian Bayan, and the Italian Giuseppe Garibaldi, though she was inferior to British designs.

The hull was constructed with transverse and longitudinal steel frames, and incorporated thirteen watertight compartments and a double bottom that extended for 57 percent of the length of the ship.

For the duration of her career as the second command flagship of the Cruiser Division, the standard crew was augmented by an additional nine officers and 44 enlisted men.

Fourteen Dürr water-tube boilers, produced by Düsseldorf-Ratinger Röhrenkesselfabrik, supplied steam to the engines at pressures up to 15 standard atmospheres (1,500 kPa).

Her armor belt was 100 millimeters (3.9 in) thick in the central citadel of the ship, which protected the ammunition magazines, machinery spaces, and other vital areas of the cruiser.

Prinz Heinrich next took part in fleet training in August and September, where she served as the flagship of II Scouting Group, with included the light cruisers Niobe and Nymphe.

After the conclusion of the maneuvers on 18 September, the pre-dreadnought battleship Kurfürst Friedrich Wilhelm was decommissioned for maintenance; the ship had been serving as the flagship of Konteradmiral (KAdm—Rear Admiral) Curt von Prittwitz und Gaffron, the deputy commander of the squadron.

While the ship was out of service, Prittwitz und Gaffron transferred to Prinz Heinrich, though he remained there only briefly before he was replaced by KAdm Ludwig Borckenhagen on 1 October.

Prinz Heinrich and the rest of the squadron ended the year with a winter training cruise, during which she tried to pull the new battleship Wittelsbach free on 17 December after she ran aground in the Great Belt.

The typical autumn maneuvers took place in August and September, during which Prinz Heinrich won the Kaiser's Schiesspreis (Shooting Prize) for excellent accuracy for large cruisers.

From 11 March to 20 June, Prinz Heinrich again served as Schmidt's flagship, as Friedrich Carl was occupied with escorting Hohenzollern on a trip abroad.

In March 1906, the ship was removed from the reconnaissance forces, her place having been taken by Friedrich Carl, which had in turn been replaced by the new armored cruiser Yorck.

Prinz Heinrich, along with Roon and a flotilla of torpedo boats, was assigned to the van of the High Seas Fleet, commanded by Admiral Friedrich von Ingenohl.

The main fleet was providing distant cover to Rear Admiral Franz von Hipper's battlecruisers, which were conducting the bombardment.

[16] During the night of the 15th, the German battle fleet of some twelve dreadnoughts and eight pre-dreadnoughts came to within 10 nmi (19 km; 12 mi) of an isolated squadron of six British battleships.

[14] On 1 July, the minelayer SMS Albatross, escorted by the cruisers Roon, Augsburg, and Lübeck and seven destroyers, laid a minefield north of Bogskär.

While returning to port, the flotilla separated into two sections; Augsburg, Albatross, and three destroyers made for Rixhöft while the remainder of the unit went to Libau.

Augsburg and Albatross were intercepted by a powerful Russian squadron commanded by Rear Admiral Bakhirev, consisting of three armored and two light cruisers.

Commodore Johannes von Karpf, the flotilla commander, ordered the slower Albatross to steam for neutral Swedish waters and recalled Roon and Lübeck.

Albatross was grounded off Gotland and Augsburg escaped, and the Russian squadron briefly engaged Roon before both sides broke contact.

[14] German naval forces in the Baltic were reinforced by elements of the High Seas Fleet during the Battle of the Gulf of Riga in early August 1915.

The battleships of the I Battle Squadron were the primary force, though Prinz Heinrich and the rest of the older vessels assigned to the Baltic fleet participated.

On 10 August, Prinz Heinrich and Roon bombarded Russian defenses at Zerel, on the southernmost tip of the Sworbe Peninsula on the island of Ösel.

A combination of tenacious Russian defense and reports of British submarines in the area—proved by the torpedoing of the battlecruiser Moltke on 19 August—caused the German navy to break off the operation.

By this time, the German navy had begun to experience severe crew shortages, and so the Reichsmarineamt decided to decommission older, less combat-capable vessels.