Giesler was also named in Hitler's Political Testament as Interior Minister, replacing Heinrich Himmler, in the short-lived Goebbels Cabinet.

His younger brother, Hermann Giesler (1898–1987), was also an architect who, along with Albert Speer, gained the favor of Adolf Hitler in Nazi Germany.

On the outbreak of the First World War, he enlisted in the Royal Prussian Army and was assigned to a pioneer battalion of the 1st Foot Guards Regiment.

Throughout this time, Giesler organized and personally participated in brutal attacks on enemies of the Party, including Communists, Social Democrats and trade unionists.

[2] However, during the Night of the Long Knives at the end of June 1934, due to being away on holiday, Giesler only narrowly escaped being arrested, and perhaps murdered, a fate that befell many other SA leaders.

Brought before the Supreme Party Court by Josef Wagner, Gauleiter of Gau Westphalia-South, on charges of planning an armed rebellion, Giesler was acquitted on the basis of insufficient evidence in April 1935.

He remained there until June 1938 when, following the Anschluss, he was charged with organizing the SA in the newly-acquired Austrian territory and was named leader of the SA-Gruppe Alpenland in Linz, a post he would hold until September 1941.

Due to his full-time Party work, he left active service with the SA in September 1941, retaining a reserve position with the SA-Gruppe Alpenland.

[6] Giesler, an unquestioning follower of Hitler, ruled Bavaria with ruthless efficiency and almost unlimited power in the last years of the war.

Giesler criticized the male students who had not been called up for military service and said that suitable employment contributing to the war effort would be found for them.

In August 1946, former SS-Obergruppenführer Karl von Eberstein, the SS and Police Leader in Munich, testified to the International Military Tribunal that he was ordered by Giesler to use his influence with the commandant of Dachau (SS-Obersturmbannführer Eduard Weiter) to have 25,000 prisoners shot when the U.S. Army approached.

At this time, he authorized some of the worst of the violence directed against so-called "defeatists" and those seeking to surrender in order to avoid further pointless death and destruction.

As the American troops approached Munich on 27 April, an uprising known as the Freiheitsaktion Bayern (Bavarian Freedom Action) broke out, led by Hauptmann Rupprecht Gerngroß who seized the radio station and called on troops to lay down their arms and arrest the Nazi leaders, and encouraged citizens to display white flags of surrender.

Having repeatedly demonstrated his fanatical loyalty to the Nazi regime, Giesler was named to replace Himmler as the Reichsminister of the Interior in Adolf Hitler's political testament of 29 April 1945, though he never actually assumed this post.

"[15] Accounts of Giesler's death vary, but the most widely accepted is that he fled Munich toward Berchtesgaden on 30 April 1945, together with his wife and mother-in-law, to avoid falling captive to American troops.

A local physician, named Gottschalk, certified Giesler's death on that date, and he was buried in the cemetery in nearby Berchtesgaden on 10 May.