Hennecke entered the Imperial German Navy (crew 15) during the First World War on 2 October 1915, as a seekadett.

From 3 October he was at the Marinschule Mürwik and then from 10 November aboard the large cruiser SMS Freya, which was used as a training ship.

He was promoted to Fregattenkapitän on 1 August 1938 and from 2 November 1938 to 30 July 1940 served as first officer on the light cruiser Nürnberg.

He was awarded the clasp of the Iron Cross 2nd Class (EK II) and was promoted to Kapitän zur See on 1 February 1940.

The German commander-in-chief West, Gerd von Rundstedt, had foreseen that the city would be an important strategic target for the Allies because of its harbour, and ordered its destruction on 9 June.

Hennecke, together with the city commander General-Major Robert Sattler and the harbour commander Kapitän zur See Witt, carried out the complete destruction of the port of Cherbourg, which began on 9 June during the Battle of Cherbourg and only ended immediately before the arrival of Allied troops in the port area on 25 June 1944.

Hennecke was first held by US forces, and was transferred on 1 July 1944 to the British prisoner-of-war camp at Trent Park.