Little is known about Captain Mastros before the outbreak. It is likely that he was already a member of the United States Armed Forces. In the aftermath, Captain Mastros and all active personnel of the US armed forces were mobilized to protect the civilian population and combat the infection.
Mastros and his battalion were responsible for maintaining the Pittsburgh quarantine zone. Over the year, they were forced to kill survivors trying to gain entry in to the zone as, in Joel's words, "you sacrifice the few to save the many".
During the final years of the zone, Mastros and his men began restricting the amount of food being given to the citizens, leading to riots occurring and many residents grafifitiing the walls of the zone.
Eventually, there was a successful uprising that drove the military out of parts of the city and led to heavy casualties. While trying to leave the city, Mastros and his surviving troops were captured and burned alive by a mob of Hunters, one soldier witnessing the event and vowing to avenge his leader.