Mack is the assistant of Rob McGroarty. Unlike the latter, he doesn't care about animals' feelings. In fact, he only cares about making money and intends to sell the animals (which Rob has been taking care of) to Tartarus Laboratories (which presumably maimed the arm of one of them, a monkey named Sebastian). Mack even teases Sebastian with a potato chip at one point. His partner-in-crime is a driver of a van for Tartarus Labs named Jim Bob. When Mack and Jim Bob are taking the animals to the lab, the former is eventually caught by Rob, and it angers him when he finds out that Mack was behind the animal cruelty. In addition, Chris (Rob's girlfriend whom Mack has been jealous of) points out that those jerks were also stealing Rob's childhood things (Toaster and the other appliances) because they were in the truck as well (when they had actually gone out to save the animals). In the end, Mack and Jim Bob are arrested by the police.
Mack is similiar to Edgar Balthazar from The Aristocats. Both are the main antagonists who love money, both are assistants to the main human character (Edgar to Madame; Mack to Rob), and both want to get rid of said character's animals to get that money. The only difference is Edgar was not evil to begin with.