Born the son of a provincial innkeeper, he was educated to doctorate level in law at Marburg University but struggled to find employment in an economically depressed Weimar Germany. He joined the Nazi Party in January 1932 and the SS the following month. With no military experience he initially floundered but soon attracted the attentions of Reinhard Heydrich, who valued academic expertise, and he was transferred to the Sicherheitsdienst (SD).
Becoming a close friend of Heydrich, Behrends was the first chief of the SD in Berlin. He also served as Chief of Staff to Werner Lorenz in his capacity as head of the Hauptamt Volksdeutsche Mittelstelle (VOMI). During the Second World War he was sent to Yugoslavia to lead the regional arm of the VOMI. His star had fallen somewhat after Heydrich's death as Heinrich Himmler was unimpressed by him, sensing that he was too ambitious.
On 5 July 1945, he faced charges from the British services in Flensburg. He was interned with the number 560294 in the Island Farm Special Camp in Bridgend, South Wales. On 16 April 1946 he was extradited to Yugoslavia. He was hanged in Belgrade on 4 December 1948.
- Louis Leo Snyder, Encyclopedia of the Third Reich, Ware, Hertfordshire: Wordsworth Editions, 1998, p. 21, ISBN 9781853266843.
- Valdis O. Lumans, Himmler's Auxiliaries: The Volksdeutsche Mittelstelle and the German National Minorities of Europe, 1933–1945, UNC Press Books, 1993, p. 50
- Lumans, Himmler's Auxiliaries, p. 50-1
- Lumans, Himmler's Auxiliaries, p. 51
- Lumans, Himmler's Auxiliaries, p. 45