At the beginning of World War II in September, she was initially deployed to lay minefields off the German coast, but was soon transferred to the Skagerrak where she inspected neutral shipping for contraband goods.

In late 1939 and early 1940, Z20 Karl Galster helped to laid three offensive minefields off the English coast that claimed one British destroyer, a fishing trawler, and twenty merchant ships.

The ship returned to Germany in early 1941 for a refit and was transferred to Norway in June as part of the preparations for Operation Barbarossa, the German invasion of the Soviet Union.

Z20 Karl Galster spent some time at the beginning of the campaign conducting anti-shipping patrols in Soviet waters but these were generally fruitless.

Z20 Karl Galster participated in the German attack (Operation Zitronella) on the Norwegian island of Spitzbergen, well north of the Arctic Circle, in September 1943.

Plagued by engine problems, the ship was under repair from November to August 1944 and then spent the next six months on convoy escort duties in southern Norway when not laying minefields.

Around March 1945, Z20 Karl Galster was transferred to the Baltic Sea where she helped to escort convoys of refugee ships and also rescued evacuees herself in May, around the time that Germany surrendered.

The details of the changes made to her anti-aircraft suite are not well documented, but photographic evidence shows that a quadruple 2 cm mount had been added by early 1944.

[6] When World War II began in September, Z20 Karl Galster was initially deployed in the German Bight where she laid defensive minefields.

[7] On the night of 17/18 October, Konteradmiral (Rear Admiral) Günther Lütjens, aboard his flagship Z21 Wilhelm Heidkamp, led Z16 Friedrich Eckoldt, Z17 Diether von Roeder, Z18 Hans Lüdemann, Z19 Hermann Künne, and Z20 Karl Galster as they laid a minefield off the mouth of the River Humber.

On the night of 12/13 November Z21 Wilhelm Heidkamp, now the flagship of the Führer der Zerstörer (FdZ) (Commander of Destroyers), Kapitän zur See (KzS) (Captain) Friedrich Bonte, escorted Z18 Hans Lüdemann, Z19 Hermann Künne, and Z20 Karl Galster as they laid 288 magnetic mines in the Thames estuary.

[8] Bonte and Z21 Wilhelm Heidkamp led a minelaying sortie to the Newcastle area together with Z16 Friedrich Eckoldt, Z20 Karl Galster, and Z22 Anton Schmitt on the night of 10/11 January 1940.

The destroyers Z14 Friedrich Ihn and Z4 Richard Beitzen were also supposed to participate, but the former had problems with her boilers that reduced her maximum speed to 27 knots (50 km/h; 31 mph) and she had to be escorted back to Germany by the latter ship.

Z20 Karl Galster and Z21 Wilhelm Heidkamp escorted the battleships Scharnhorst and Gneisenau as well as the heavy cruiser Admiral Hipper during the sortie on 18 February.

In June the ship was tasked to lead the escort force for Scharnhorst, Gneisenau, and Admiral Hipper during Operation Juno, a planned attack on Harstad, Norway, to relieve pressure on the German garrison at Narvik.

The British opened fire at extreme range and were forced to disengage in the face of long-range torpedo volleys and attacks by Luftwaffe bombers without having hit any of the German ships.

On the night of 24–25 November, Z20 Karl Galster, no longer Berger's flagship, Z4 Richard Beitzen and Z10 Hans Lody sortied from Brest, bound for the Land's End area.

The destroyers sank a small survey ship and a flying boat and they were repeatedly attacked by Soviet aircraft to no effect before their return on the 24th.

Z20 Karl Galster was now assigned to escort convoys between Tromsø and Kirkenes; during one of these missions, the submarine Trident sank two troop-carrying freighters, Bahia Laura and Donau II despite the destroyers.

The ships sailed from Trondheim on 2 July for the first stage of the operation, although three of the destroyers, including Z20 Karl Galster, assigned to Tirpitz's escort ran aground in the dark and heavy fog and were forced to return to port for emergency repairs.

Several weeks later, Z20 Karl Galster, and the destroyers Z5 Paul Jacobi and Z6 Theodor Riedel, sailed for Jan Mayen island on 31 March to rendezvous with the blockade runner, MV Regensburg.

[17] Problems with her starboard turbine, after the overhaul was finished in April 1944, put Z20 Karl Galster back in the dockyard for further work until about August when she began escorting convoys in southern Norway and helping to lay minefields in the Skagerrak.

There the ship received an open radio transmission by Grand Admiral (Großadmiral) Karl Dönitz asking "everybody to "Curry" who can make it until dawn on the 8th".

The German captains knew "Curry" to be the nickname of their former naval academy instructor, Admiral August Thiele, commander in Hela on the Hel Peninsula.