At some point in his life, Mackmain became an agent of SPECTRE, and was part of an Eidolon unit, designed to stock supplies for SPECTRE's return.
Some time before the events of VARGR, Mackmain worked to enforce the "Hard Rule", a law put in place to make it so that no MI6 operatives could carry firearms inside the United Kingdom.
When a conference about the Hard Rule was proposed, Mackmain attempted to charm and bribe some influential voices in the argument to keep it in place. Eidolon however, had been discovered, and this necessitated him scheming with his Eidolon cohort Beckett Hawkwood to eliminate the opposition to the Hard Rule right out. Mackmain met with the Commissioner of Defense and the head of MI6. During the conversation, MI6 revealed that they knew of the Eidolon cell operating within MI5, and the Commissioner fired Mackmain, who proptly killed him by shooting him in the head. Hawkwood then entered and prepared to shoot the head of MI6, but James Bond crept into the room and shot Mackmain directly in his spine. Bond then walked over and killed Mackmain by shooting him again, but in the head.