Very little is known about Mikey's life before the events of The Goonies. When Mikey was younger, his father used to tell him about One-Eyed Willy, which inspired him to one day seek out Willy's treasure. Mikey led The Goonies on a treasure hunt for pirate treasure from the 1600s. On a rainy day, Mikey and his friends went up into the attic of their home and found a map leading to One-Eyed Willy's treasure.
Mikey has asthma so throughout the movie he is shown with his inhaler. At the film's end, he discards the inhaler, feeling he no longer needed it.
Mikey has trouble pronouncing words in which Brand has to correct him. He says dictate when he meant to say delude, says retropactum when he meant to say retrospective, and calls his dad the museum curny or curly when he meant to say curator.
Mikey also has braces and tells Mouth to stop messing with Chunk.
Mikey used his inhaler nine times during the film.