The boy did not have a gun, never issued any tickets and didn't drive the squad car, Deputy Superintendent Daniel Dugan said.
Assistant Superintendent James Jackson said the ruse was discovered only after the boy's patrol with an actual officer ended Saturday. Officers noticed his uniform lacked a star that is part of the regulation uniform.
Police said they were investigating how the deception went undetected for so long in what they described as a serious security breach.
Police didn't identify the boy because of his age. He has been charged as a juvenile with impersonating an officer.
Dugan said the boy looks older than 14 and was motivated by a desire to be an officer, not malice or "ill intent."
The boy once took part in a Chicago program for youth interested in policing, so he would have been familiar with some procedures, perhaps helping him blend in, police spokeswoman Monique Bond said.