Mack is found first in Princess Peach's Castle within Mushroom Kingdom. After his battle with Croco, Mario finds the Mushroom Kingdom in a state of turmoil. Many of the citizens of the town are being terrorized by Shy Guys all around. Mario, with the help of Mallow defeat most of the Shy Guys and make their way to the castle. At the castle, they find Mack who has led the siege on the Mushroom Kingdom.
Mack also appears during the ending credits sequence of Super Mario RPG. Durign the credits parade, Smithy and his three main henchmen appear where Smithy smashes all three with his fist due to their failures in defeating Mario.
The battle with Mack takes place within the castle throne room where Mario must battle his way through several Shysters in order to get to Mack. During the battle, Mack summons up to four Shysters simultaneously. Mack also unleashes different fire-based attacks, implying that Mack is a fire-based enemy.