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.