In 1929 he joined the Nazi Party and became head of Organization Department II that same year. In the federal election of 1930, he became a member of the Reichstag parliament. On 5 June 1931, two years before the Nazi Party ascended to national power, Hierl became head of the FAD, a state sponsored voluntary labour organization that provided services to civic and agricultural construction projects. There were many such organizations in Europe at the time, founded to provide much-needed employment during the Great Depression. and Hans Frank at a diplomatic reception, Berlin, February 1939 Hierl was already a high-ranking member of the NSDAP when the Party took power in January 1933. He remained the head of the labour organization - now called the Nationalsozialistischer Arbeitsdienst, or NSAD. Adolf Hitler named him as State Secretary in the Reich Ministry of Labour under Franz Seldte, with the order to build up a powerful labour service organization. Facing Minister Seldte's resistance, Hierl in 1934 switched to the Reich Ministry of the Interior under Wilhelm Frick in the rank of a Reichskommissar. On 11 July 1934, the NSAD was renamed Reichsarbeitsdienst or RAD 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 and one for women. The RAD was composed of 40 Gau-sections. In 1936 the Reich Labor Service built the model village of Hierlshagen, named after Hierl. He was named Reich Labor Leader in 1935 and Reichsleiter, the second highest political rank in the Nazi Party, on 10 September 1936. Also in 1936, he was awarded the Golden Party Badge. Hierl was further appointed Minister Without Portfolio in 1943. '', 19 October 1941 During World War II, hundreds of RAD units were engaged in supplying frontline troops with food, ammunition, repairing damaged roads and constructing and repairing airstrips. RAD units constructed coastal fortifications, laid minefields, manned fortifications, and even helped guard vital locations and POW camps. The role of the Reich Labor Service was not limited to combat support functions. Hundreds of RAD units received training as anti-aircraft units and were deployed as Flak batteries. On 24 February 1945, Hierl was awarded the German Order, the highest decoration the Nazi Party could bestow on an individual. After the war, he was tried and found guilty of "major offenses". Hierl was sentenced to five years in a labour camp. Following his early release, he lived in Heidelberg until his death on 23 September 1955.