Karl Weinrich was a member of the Nazi Party from August 1922. From 1925 to 1927 he was the NSDAP's Gau Treasurer. From 1930 to 1933 he was a member of the Prussian Landtag, and as of 12 November 1933 a member of the Reichstag for the electoral district of Hesse-Nassau. He was from 1928 to 1943 Gauleiter of Kurhessen. Shortly after the bomb attack of 22 October 1943 on Kassel, which destroyed the whole inner city, he was stripped of his office. His successor was Karl Gerland.