He was the last of three officers to be appointed to the rank of General of air force communications troops.

Fahnert entered the Imperial Army as an officer cadet in the 2nd railway regiment, Berlin in 1900.

At the outbreak of World War II, he was the commander of the Air Signals Training School in Berlin.

He then commanded the Air Signals Training Division in France and was promoted to General der Luftnachrichtentruppe on 1 April 1945.

On 2 May 1945, Fahnert was taken into British captivity in Lübeck and then held with other senior officers at Special Camp XI near Bridgend, South Wales until 5 December 1947.