According to Schellenberg, Mehlhorn was "in a state of shock" when ordered by Heydrich, whom he considered his enemy, to lead the Gleiwitz operation. Schellenberg advised Mehlhorn to ask Heydrich to excuse him from taking part in the planned attack due to physical inability; Mehlhorn followed Schellenberg's advice and was surprised when Heydrich dismissed him from the ranks of the SD without raising any issues. In 1939, he was sent to Posen where he joined the CdZ after his relation with Heydrich deteriorated.