Lehmann was found guilty of war crimes at the High Command Trial at Nuremberg in 1948.

He grew up in Breslau and Hanau, studied law in Munich, Freiburg, Leipzig and Marburg and qualified as a lawyer before service as a reserve officer in the Imperial Army in World War I, during which he was awarded the Iron Cross.

In this role he assessed the charges against Generaloberst Werner von Fritsch between February and March 1938 and his judgement was for an acquittal.

On 1 May 1944 he was given the unique rank of Generaloberststabsrichter (Colonel General Judicial Officer) which was equivalent to Generaloberst but outside the normal chain of command.

He was found guilty of war crimes at the Nuremberg High Command Trial in October 1947 for involvement in drafting the Barbarossa decree and the Commissar Order that stipulated that any captured Soviet political commissar was to be executed, contrary to international law.