Unlike his fellow minions (as far as we know anyway), Mike was too deep within Gru's secret lair, down in the mailroom, to have been captured and turned into an evil minion. Until, one day, a package came down the automated conveyor belt containing a shipment of the dangerous mutagenic serum PX-41, which was probably meant for Dr. Nefario's attention (shortly before he resigned from Gru's employ in order to secretly work for El Macho).
When Mike marked the package for the appropriate routing tube, and sent it along to Ken, the bored co-worker decided to have a little fun, and toss the box into its routing tube from a distance. This resulted in it getting stuck in the tube. When Mike saw what happened, and how the tube's pressure was backing up (and that it was about to blow), he sharply reprimanded Ken and then went over to try and free the package. Suddenly, under pressure, the package buckled, and some of the PX-41 serum sprayed onto poor Mike. The result was his transforming, sporadically and repeatedly, into an evil minion.
It is only because he wasn't doused with more of the serum, and that he didn't ingest it, that Mike eventually managed to shake it off and return to normal. But not before bringing a short run of chaos and terror to the otherwise quiet little mailroom!
(The evil minions are rather more dimwitted than the normal yellow minions who work for Gru. They also have an underslung jaw with a heavy underbite, over-sized fangs, massive unkempt hair [and a generally hairier body], scraggly chin hair, and oversized, gangly arms. They cannot communicate in the normal minion language, though they appear to understand it. Their most frequent utterance is a short, abrasive and intimidating "BAAH!". They constantly twitch and shudder, and mumble and grunt. They are also practically invulnerable to harm [even from themselves]. They can be shot by gunfire, exploded by bombs, have massive heavy items dropped on them, flames hurled at them, and still more, and yet bounce right back with no ill effects. They can also eat anything, regardless of its composition...even explosives. Their only motivation seems to wreak havoc. The evil transformed minions were, it turns out, created by Dr. Nefario, when he briefly went to work for supervillain El Macho...until being saved by him when he had a change of heart and returned to work for Gru. All of them returned to normal...apparently except one!)