In 1933 he headed the Externsteine-Stiftung ("Externsteine Foundation"), which had been founded by Heinrich Himmler to study the Externsteine in the Teutoburger Wald. In 1935, having joined the SS that year, Sievers was appointed Reichsgeschäftsführer, or General Secretary, of the Ahnenerbe, by Himmler. He was the actual director of Ahnenerbe operations and was to rise to the rank of SS-Standartenführer by the end of the war.