Background & Early Life
Little is known about Captain Mastros before the outbreak. At some point, he joined 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 was a military officer in the Pittsburgh Quarantine Zone. In the initial outbreak, Mastros and his forces 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".
Over time, Mastros' soldiers began restricting the amount of food being given to the citizens, leading to riots occurring and many residents spraying graffiti on the walls of the zone.
Eventually, there was a long uprising that drove Mastros and the military out of parts of the city and led to heavy casualties. In the closing stages of the 14-month rebellion, Mastros reported to his superior officer, Colonel Mackenzie, who was located in Atlanta Quarantine Zone, to request permission to abandon the zone.
However, before Mastros could send the note out to his superiors, he and his surviving troops were captured and burned alive by a mob of rebels, save one soldier who witnessed the event and vowed to avenge his leader.
Events of The Last of Us
In The Last of Us, Captain Mastros is mentioned in two artifacts that Joel finds in Pittsburgh, the "Abandon Zone Note" that Mastros wrote himself, and in the "Mob Attack Note" which describes his demise.