Cormac worked on a Prison Ship as a Guard.
Biography
Begining to work on the Prison Ship
At some point before the events of DWA:About Face, Cormac began his work as a guard on a Prison Ship.
Being Possessed
Shortly after a Charonoid (a creature than can switch bodies with other beings) escaped from a cell, the Charonoid switched bodies with Cormac. The Charonoid used a Stun Pistol to knock out the other guards. (DW: About Face)
Meeting The Eleventh Doctor
Shortly after knocking out the guards, Cormac (in the Charonoid's body) met the Doctor and Amy Pond, but was shot at by the Charonoid (in Cormac's body). The Charonoid stated that he is Cormac, but when The Doctor and Amy found the bodies of the guards, The Doctor realised the truth. The real Cormac ran in and fell into The Doctor's trap for the Charonoid. Cormac became free from the Charonoid's body when the Charonoid swapped bodies with Amy. The Doctor tricked the Charonoid into freeing Amy and returning to it's own body. The Guards probably woke up and Cormac probably continued to be a guard for the ship. (DWA: About Face)
Personality
When the Charonoid was in Cormac's body, he seemed to be an aggressive guard, but after he was freed, he appeared to be calm.