He was awarded the Knight's Cross of the Iron Cross for service in the German retreat from France in 1944. He was the commander of the defense of the central government area in the Battle of Berlin, 1945.
Mohnke led the 5th company of the 2nd Battalion of the Infanterie-Regiment Leibstandarte SS Adolf Hitler, at the outset of the Battle of France in 1940. He took command of the 2nd Battalion on 28 May after the battalion commander was wounded. It was around this time that Mohnke was allegedly involved in the murder of 80 British prisoners of war (POWs) of the 48th South Midland Infantry Division.
Mohnke was noted to be one of the last commanders to act upon orders issued by Hitler himself prior to his suicide.