At the beginning of the Second World War, he was given command of SS-Sturmbann Nº 13, a unit of the SS Standarte Deutschland.

In November 1942, Grothmann helped organize Operation Harvest Festival, the largest single massacre of Jews by German forces during the Holocaust.

[1] During the last few days of the war in Europe, Himmler, Grothmann and Heinz Macher traveled from Lübeck to Flensburg, where Himmler offered his services as second-in-command to the Flensburg government led by Karl Dönitz, successor to Adolf Hitler.

Himmler had not made extensive preparations for this, but he had equipped himself with a forged paybook under the name of Sergeant Heinrich Hitzinger of the Geheime Feldpolizei (Secret Field Police), which was a mistake since members of this organization were sought after by the occupation forces.

Grothmann served as a prosecution witness against several SS officials between 1946 and 1948,[6] but during the trial of Karl Wolff he denied having any knowledge of the Final Solution.

After release from Allied internment, Grothmann was denazified, considered part of category III (Lesser Offenders), by a court in Freising in March 1949.