Mike is the most hated child in Derry by Henry Bowers, who developed this hatred by paralleling his father's hatred of Mike's father, Will. Henry terrorizes Mike when he gets a chance, and he killed Mike's dog, Mr. Rat, by Chips poisoning.
While checking out the remains of the Kitchener Ironworks, Mike encounters It in the form of a giant bird. Mike hides in a smokestack and survives.
Mike is the last to join the Losers Club, bringing them to lucky number 7. While being chased by Henry and his friends Victor "Vic" Criss, Patrick Hockstetter, Reginald "Belch" Huggins, Moose Sadler and Peter Gordon, Mike is eventually trapped in a gravel pit. He tries to hold off Henry, but the bully throws an M-80 at him. The Losers Club comes to his rescue in what comes to be known as The Apocalyptic Rockfight. The Losers beat Henry's gang, and Mike becomes a member of their group.
Mike brings his father's photograph album to the Barrens to show his new friends that It shows up in many of the antique pictures, proving that It has been part of Derry for a long time.
When The Losers use their clubhouse to try an old Indian ritual to get a vision, it is Mike and Tichie Rozier that see It landing on Earth from beyond space in time long ago.
Mike recognizes mooseblowers when The Losers go to Neibolt Street with the silver slugs. This is a sign that It is also afraid of them.
Mike is the only one of the Losers to stay behind in Derry (and thus the only one to retain his memory of the events of 1958) and becomes the town librarian. He recalled the rest when the murders begin again in 1985. His father kept an album filled with photos that were important to Derry's history, including several of Pennywise the Clown. Through the knowledge he acquires of Derry and It, he becomes an amateur historian of the town. He is seriously wounded by Henry Bowers and nearly dies. He is not part of the final battle against IT because of his injuries. He is almost wounded by a man who was under the control of IT while in the hospital. He later recovers from his wounds, but, like the others, starts to lose his memory of the experience.
- "Thanks, Guys. But, you shouldn't have done that. He'll be after you too now."
- ―Mike to the Losers Club
- "Maybe. Or maybe IT knows what scares us most and that's what we see."
- ―Mike to the Losers' about IT knows about their fears
- "You know the that burnt down house on Harris Avenue? I was inside when it burned down. Before I was rescued, my mom and dad were trapped in the next room over from me. They were pushing and pounding on the door trying to get to me. But it was too hot. When firemen finally found them, the skin on their hands melted down to the bone. We're all afraid of something."
- ―Mike reveal his past about the Harris Avenue burnt down house and his parents' deaths