McGillion is a terran marine sergeant in the Annihilators, serving as the head of E Company. Initially, he had always wanted to be a doctor, but he washed out of medical school. He then enlisted in the Confederate Marine Corps to avoid the ridicule of his family. Passionate about sports teams such as the Tarsonis Tigers, he will often rant on various sport-related topics to other members of the unit.
In the weeks leading up to the fall of the Confederacy, McGillion was heavily involved in the war against the zerg, only to be recalled to Tarsonis to help recover a rogue telepath for the Ghost Program. Along with wrangler Malcolm Kelerchian, McGillion and the rest of the unit were dispatched to the southwestern district of Tarsonis City. Due to orders from General Ledbetter, the plan was accelerated, McGillion leading E Company, alongside Major Esmerelda Ndoci (the unit's CO), straight against the headquarters of of crime lord Fagin, in which the subject was located. McGillion located the target, though soon found himself fighting the zerg.
In the wake of the fall of the Confederacy, McGillion retained his position, though now in the service of the Dominion Marine Corps.