During the Second World War, he served as a Higher SS and Police Leader (HSSPF) in Nazi-occupied Poland, the Soviet Union and the Protectorate of Bohemia and Moravia.

He marched his SA unit through Nuremberg in support of the failed Beer Hall Putsch then taking place in Munich in November 1923.

He rejoined the legalized Nazi Party on 1 June 1928 (membership number 89,221), becoming a member of the Ortsgruppe (Local Group) in New York.

Hildebrandt remained in this post until 1 October 1932 when he succeeded Dietrich as the commanding officer of SS-Gruppe Süd in Munich, where he served until 30 January 1933.

He was subsequently elected as a deputy from electoral constituency 19, Hessen-Nassau, on 29 March 1936 and retained this seat until the fall of the Nazi regime.

[5] On 21 September 1939, Hildebrandt was named the first HSSPF "Weichsel," which was made up of Danzig and those areas annexed from Poland that were formed into the Reichsgau Danzig-West Prussia.

[6] Almost immediately upon taking command, Hildebrandt began enforcing the Nazi racial policies including persecution of Jews and ethnic Poles.

Hildebrandt's area of jurisdiction was also the site of the Intelligenzaktion Pommern actions, in which members of the Polish intelligentsia were systematically murdered.

[7] Hildebrandt was also the deputy for the "Weichsel" area to Reichsführer-SS Heinrich Himmler in his capacity as Reich Commissioner for the Consolidation of German Nationhood (RKFDV).

[8] In addition to his staff position, Hildebrandt returned to an active SS and police command when he replaced SS-Oberführer Heinz Roch as the SSPF in "Taurien-Krim-Simferopol" on 25 December 1943.

From this time, he also functioned as Himmler's Chief Liaison Officer to the Commander-in-Chief of Army Group Center, Generalfeldmarschall Ferdinand Schörner.

In April 1945, he received his last posting, succeeding SS-Obergruppenführer Karl Hermann Frank as HSSPF "Böhmen-Mähren" in the Protectorate of Bohemia and Moravia.

[9] After the end of the war in Europe, Hildebrandt lived in Wiesbaden under an assumed name until he was discovered and arrested by the Americans on 24 December 1945 and interned in Regensburg.

He stood trial from 8 October to 4 November 1949 for crimes committed during his tenure as HSSPF in Weichsal, together with SS-Brigadeführer Max Henze who had been the Chief of Police in Bydgoszcz and Danzig.