Peter Mills, played by Charlie Barnett on Chicago Fire, left the show at the end of Season 3 because the writers felt his character had reached a natural stopping point in his story. In the show, Mills decided to leave Firehouse 51 and move to North Carolina to help his family with their restaurant business. The decision to write Mills off was a creative choice made to introduce new characters and fresh storylines to the series, not due to any issues with the actor or his performance. The showrunners wanted to keep Mills' departure respectful and left the door open for a possible return, as he fulfilled his character's arc of honoring his late father's legacy as a firefighter. This exit was seen as a meaningful, thoughtful conclusion to his character's journey rather than a sudden departure.