In January 1940 she was used as a commerce raider in the Baltic Sea and later took part in the invasion of Norway; as a command ship of a transport squadron.

Although Brummer was primarily designed for anti-aircraft gunnery training, she was also fitted for mine laying, her intended primary duty during wartime.

Brummer was also used as an experimental ship for the high pressure steam turbine systems designed for the German destroyers.

After being commissioned in 1936 Brummer worked up in the Baltic Sea, and was then attached to the Naval Air Defense and Artillery School (German: Marineflugabwehr- und Küstenartillerieschule) in Swinemünde in the spring of 1937.

In April 1940, Brummer took part in Operation Weserübung, the invasion of Norway; as a command ship of a transport squadron.