Hierl was born in Parsberg near Neumarkt in the Bavarian Upper Palatinate region, and attended secondary school (Gymnasium) in Burghausen and Regensburg.

During World War I, Hierl served as a member of the general staff of the I Royal Bavarian Reserve Corps, part of the German 6th Army fighting on the western front.

Upon the German defeat and the November Revolution of 1918, Hierl became head of a paramilitary Freikorps unit and fought in Augsburg against supporters of the Spartacist uprising.

He was discharged from the service with the rank of Oberst in September 1924, in part due to his support of General Erich Ludendorff's participation in the Beer Hall Putsch.

[3][4] In April 1929, Hierl joined the Nazi Party (membership number 126,752) and became head of Organization Department II that same year, serving as deputy to Gregor Strasser.

[8] On 11 July 1934, the NSAD was renamed Reichsarbeitsdienst or RAD (Reich Labor Service), which Hierl would control as its chief until the end of World War II.

The Reich Labor Service was divided into two major sections, one for men (Reichsarbeitsdienst Männer - RAD/M) and one for women (Reichsarbeitdienst der weiblichen Jugend - RAD/wJ).

Hierl was named Reich Labor Leader (Reichsarbeitsführer) on 26 June 1935 and Reichsleiter, the second highest political rank in the Nazi Party, on 10 September 1936.

[8] In August 1943, when Heinrich Himmler replaced Frick as Interior Minister, Hierl also left the ministry and reported directly to Hitler as the head of a Supreme Reich Authority (Oberste Reichsbehörde).