For most of World War I, Erzherzog Karl remained in her home port of Pula, in present-day Croatia, except for four engagements.

Her sole combat engagement occurred in late May 1915, when she participated in the bombardment of the Italian port city of Ancona.

She also took part in suppressing a major mutiny among the crew members of several armored cruisers stationed in Cattaro between 1–3 February 1918.

She also attempted to break through the Otranto Barrage in June of that year, but had to retreat when the dreadnought SMS Szent István was sunk.

[4] The ships had a tertiary armament for protection against torpedo boats in the form of the 7 cm (2.8 in)/45 caliber gun, also manufactured by Škoda.

[1] A major mutiny among crews of the armored cruisers stationed in Cattaro, including Sankt Georg and Kaiser Karl VI, began on 1 February 1918.

[8] For the morning of 11 June, Admiral Miklós Horthy planned a major assault on the Otranto Barrage; the three Erzherzog Karls and the four Tegetthoff-class battleships were to provide support for the Novara-class cruisers.

[9] This was to be the last military action the Erzherzog Karl-class ships were to take part in and they spent the rest of their career at port in Pula.