Jimmy McCoy, 27, "probably thought he was helping" when he took off in the ambulance Saturday, Fire Department spokesman Larry Langford said in a Chicago Sun-Times report.
McCoy said he "thought he could get to the hospital quicker," as paramedics worked on a relative in the rear of the ambulance, the report said.
The male and female paramedics on the ambulance radioed in, saying: "This is ambulance 23. Our ambulance has been stolen, and we are in the back with a patient,'' Langford said.
A fire truck dispatched along with the ambulance was able to block an intersection just three blocks away, where McCoy was arrested and charged with felony unlawful possession of a stolen motor vehicle, the newspaper said.
The relative was being treated for a diabetic condition, and was taken to a hospital in another ambulance; no one was injured in the incident, officials said.