Z21 Wilhelm Heidkamp later helped to lay four offensive minefields off the English coast that claimed two British destroyers, 2 fishing trawlers, and twenty-seven merchant ships.

[1] During Z21 Wilhelm Heidkamp's sea trials, she reached 37 knots (68.5 km/h; 42.6 mph) from 69,950 PS (51,450 kW; 68,990 shp), but a full-speed test was never conducted.

After working up, Z21 Wilhelm Heidkamp became the flagship of the Commander of Torpedo Boats (Führer der Torpedoboote) Rear Admiral (Konteradmiral) Günther Lütjens and patrolled the Skagerrak to inspect neutral shipping for contraband goods in late September.

[5] On the night of 17/18 October, the ship 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 Commander of Destroyers (Führer der Zerstörer), Captain (Kapitän zur See) Friedrich Bonte, escorted Z18 Hans Lüdemann, Z19 Hermann Künne, and Z20 Karl Galster as they laid 288 magnetic mines in the Thames Estuary.

Less than a week later, Z21 Wilhelm Heidkamp, Z19 Hermann Künne, and Z11 Bernd von Arnim laid 180 magnetic mines in the Thames Estuary on the night of 17/18 November.

[6] After a refit in Stettin between 27 November and 24 December, 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.

[8] Z21 Wilhelm Heidkamp was the flagship for Group 1, commanded by Commodore (Kommodore) Bonte, for the Norwegian portion of Operation Weserübung in April 1940.

Although Z21 Wilhelm Heidkamp's stern was below water, her captain, Lieutenant Commander (Korvettenkapitän) Hans Erdmenger, managed to moor her to a nearby Swedish freighter.