Captain Mohler was apparently the ranking officer at the Nazis' island base near Crete. Prior to his arrival, Belloq had radioed Mohler and told him to set up an altar for the Ark-opening ceremony. He and Belloq had made a deal whereby Mohler would arrange the Jewish ceremony, in return for Belloq speaking favorably of him once back in Berlin. He met Belloq and Dietrich at the base when they arrived, then walked with them, Toht, the captive Marion Ravenwood and several soldiers to the altar. He was presumably killed when Belloq opened the Ark and unleashed the wrath of God.
Mohler is identified as simply Tall Captain in the script and film credits.