After being born to abusive parents in County Cork Ireland, Sean McCallahan moved to the United States aged 18 and joined the United States Marine Corps. After basic training at the Marine Corps Camp of Infantry Training West, he was deployed to Afghanistan to suppress rebel movements and establish peace in the area. After spending 4 years in Afghanistan and being deployed to a base back home, Sean joined the United States Marine Corps Military Police Division and underwent basic training at the same school of infantry as before. After basic training, Sean was deployed to Base Ules in Chernarus to work base and gate security. While still deployed at this base, they received word of the outbreak spreading and Sean and his MPD team were instructed to escort and guard citizens in a hospital located in the city of Chernogorsk.
While there, the hospital was quickly overrun by infected citizens from the outside. After the hospital was overrun Sean and his team moved back to a multi-story apartment building to fend off the rest of the citizens they managed to evacuate. Sean sent his team and the other citizens to go to a more secure location while he held the infected citizens at the apartment building. This didn't last long however, as the building quickly filled with the infected. Sean ran to the nearby wood in search of a place to hide through the afternoon while he attempted to make contact with his team.
Little did Sean know, his team and citizens had been stopped by a band of insurgents and were sadly executed on the side of the road because the insurgents believed the United States were trespassing on sacred land. After spending the night in the wood, Sean decided to move on to a nearby village, where he dropped his gear and grabbed other clothing so as to avoid detection from insurgents which he had come into contact with while in the woods. Sean's story still continues on to this day.