Johnen enlisted in Luftwaffe in 1939 and in 1941 he joined the German night fighter force (Nachtjagd) and participated in the Defence of the Reich campaign.
He became a night fighter ace in June 1943 after achieving his fifth victory. In April 1944 Johnen was briefly interned in Switzerland when he force-landed in the country. It caused an international incident but he was repatriated with his crew some days later.
Johnen was awarded the Knight's Cross of the Iron Cross in October 1944 for the destruction of 33 RAF bombers. He ended the war with 34 aerial night victories. After the war he worked for Willi Messerschmitt and started his own company. He died in 2002.