Serving as a gunner in the German Air Force, Hennrich was assigned to KG II on 20 April 1916. In February 1917, he was selected for pilot training and was posted to FA(A) 273 on 10 October 1917. Joining Jasta 46 on 6 May 1918, Hennrich became the highest scoring ace to serve with this unit. Flying the Fokker D.VII, he destroyed 13 enemy balloons, finishing the war with 20 victories.