[1][2] After the end of the war in 1918, he obtained an entry-level position in the Prussian Ministry of Trade and Industry, becoming a Staatskommissar (State Commissioner) in July 1920 and being promoted to Ministerialrat (Ministerial Counselor) in 1921.

On 8 May 1934, on the retirement of Bruno Claußen, Posse additionally was charged with management of the affairs of the State Secretary in the Prussian Ministry of Economics and Labor.

After the outbreak of the Second World War, Posse in early 1940 also became head of Special Staff I (Economy) in the High Command of the Army and, in 1941, he was appointed Reichskommissar for liaison with the Unilever group, located at Rotterdam in the occupied Netherlands.

[1][2] After the end of the war in Europe, Posse was held by the American military in Neumünster and interrogated on 12 April 1946 in preparation for the Nuremberg trials where Funk, his former superior, was to face charges.

[3] According to Funk's rebuttal testimony at Nuremberg, Posse technically was his chief deputy as General Plenipotentiary for the Economy, but had no real responsibilities, in contrast to Friedrich Landfried (who had succeeded Brinkmann) at the Ministry and Emil Puhl at the Reichsbank: "Mr. Posse was an old, sick man whom I had assigned to this post … which, to all intents and purposes, existed only on paper.