He was first called in to voice Mickey and understudy with Wayne Allwine, but unfortunately, Wayne passed away from heart failure and diabetes-related complications on May 18, 2009 and they never had the chance to meet each other. After Allwine's death the role of Mickey was given to Iwan automatically.
Bret first recorded Mickey Mouse dialogue for the Animal Kingdom theme park as well as the 2009 shows Disney On Ice: Celebrations and Disney Live: Rockin' Road Show. He gave his first full performance as Mickey Mouse for the English version of the PlayStation Portable game Kingdom Hearts: Birth by Sleep. He gave his voice performance as Mickey Mouse in the Epic Mickey games.
His first voice-over work in a Disney park could be heard in the Animal Kingdom closing show "Adventurers' Celebration Gathering" as well as on the Tomorrowland Transit Authority PeopleMover attraction at the Magic Kingdom, in which upon passing through Mickey's Star Traders, Mickey responds with his signature laugh and says "That's right, it's outta this world!"
Bret Iwan has stated that in the third season of Mickey Mouse Clubhouse, the voice of Mickey was still indeed Wayne Allwine, due to Mickey's voice sounding a bit lower in the later episodes (Wayne had already recorded Mickey's voice for many of those episodes before his death). Wayne would change his Mickey voice to a mid-low pitch to make Mickey sound more calm as he spoke. Viewers thought that Iwan had already taken over the role when the new episodes came out after Allwine's death. Unlike Allwine, Iwan is keeping Mickey's voice in a higher lighter range similar to what Allwine would normally do when he used to voice Mickey before Mickey Mouse Clubhouse.
Bret's father, Bill Iwan, has an O2½ model railroad that was featured in the April 2011 Model Railroader.