In December 2499, Private Mellish deployed to Mar Sara as part of Third Squad, 417th Confederate Marine Platoon. The platoon was assigned to retrieve a piece of Confederate equipment. En route to Oasis, Mellish became nervous as the unit entered zerg-infested territory and faced with the prospect of walking on creep and even before that, had no idea what the zerg were nor did he even hear anything about them before until Lieutenant Breanne informed him.
Mellish survived the battle against the zerg at Oasis and the retreat back to Bunker Complex 3847 where he, Bernelli, Cutter, and Xiang were initially tasked with securing the perimeter. He was however, the only survivor of Third Squad in the battle. When the extraction transport crashed the platoon opted to fight and draw zerg attention away from civilians being evacuated by the Sons of Korhal. Mellish presciently noted that the Sons of Korhal were not interested in the civilians any more than the Confederacy was and that both were ultimately different sides of the same coin.
Mellish helped to fortify the bunkers with mines. He was likely killed when the group began its retreat to the last bunker as he was not among those in the final stand against the zerg.
Mellish's comments on the Sons of Korhal not being interested in civilians any more than the Confederacy would later be correct, given that the Sons of Korhal leader Arcturus Mengsk later ordered the use of psi emitters luring billions of zerg to the Confederate capital world Tarsonis with no regard of the civilian population.