Jost joined the Nazi Party on 2 February 1928 with an NSDAP membership number of 75,946. He performed various functions for the party's operations in southern Hesse. From 1930 he settled as an independent lawyer in Lorsch, Hesse. After the Nazi seizure of power in March 1933, Jost was appointed Director of Police in the city of Worms and then to police director of Giessen. From this period came his association with Werner Best, who brought Jost into the main Nazi intelligence and security agency, the "Sicherheitsdienst" (SD). On 25 July 1934, Jost began his full-time career with the SD. His SS membership number was 36,243. In May 1936, Jost was promoted in the SD Main Office to head Department III 2 (Foreign Intelligence Services). In 1938, Jost was head of the "Einsatzgruppe Dresden" which occupied Czechoslovakia. In August 1939 Jost was tasked by Reinhard Heydrich with obtaining the Polish uniforms needed for the false flag attack on the station in Gleiwitz.