Janzen transferred to the German Air Force on 4 May 1916. After training with FEA 3 and Kasta 12, he was assigned to Jasta 23 on 28 November 1916. He scored his first victory with this unit before being reassigned to Jasta 6 on 16 October 1917. After scoring 3 more victories, he assumed command of Jasta 4 on 28 March 1918. A week later, he returned to Jasta 6 as the new CO, replacing Wilhelm Reinhard who was given command of JG I. Scoring his 5th victory on 4 May 1918, Janzen's Fokker DR.I was shot down five days later by a member of 209 Squadron. He survived the crash and scored 8 more victories before he was shot down again on 9 June 1918. On this occasion, the interrupter gear on his triplane malfunctioned during a dogfight with a SPAD and he shot off his own propellor. Janzen was captured and remained a prisoner until the end of the war.