Johnson drove home early Thursday when he felt dizzy and parked his car near his home, the police said in a statement.
Officials found him looking for a passer-by called 911 to report that a person had fallen asleep at a stop sign.
"The appealing officers arrived at the scene and checked the well-being of the superintendent, the officers did not notice any signs of impairment, and the Superintendent drove home," the police said. Johnson was not given a sobriety test.
"He revealed that he drank some drinks for dinner," said Lightfoot, noticing that she did not explicitly ask Johnson if he had been drinking.
When CNN reached the mayor's office, her staff referred to the newspaper's interview.
Johnson told reporters Thursday that he feels his blood pressure has risen when he decides to shut down his car.
Johnson said he had been prescribed a drug after suffering a blood clot in the summer. He thinks he felt ill on Thursday because his doctor had changed his medication earlier in the week, but he accidentally missed it.
The police reported that he went to see a doctor on Thursday for problems with his blood pressure.
He was not asked to perform a sobriety test.
At the request of Johnson, an internal investigation of the incident is conducted.
"The Superintendent initiated this investigation because, as he put it," whether you are a police officer or a superintendent, all officers should be kept to the highest standard, "the police said in a statement.
Than Asked by the Sun-Times why the Superintendent did not receive a field sobriety test, Lightfoot did not want to "prejudge" the internal investigation.
"I know what the Superintendent told me he had some medical problems this week , Medicines changed, did not feel well, withdrew to the side. And then the IAD will do the rest. Johnson told reporters on Thursday that officials did not feel the need to do the sobriety test.
"When somebody sleeps in a car, that does not mean he's in any way impaired," Johnson said
] A Chicago police spokesman said there was no evidence of misconduct.
"We have no evidence of inappropriate behavior at this time, but this question can only be answered by the internal investigation. This investigation [is] is active and still ongoing, "said the spokesman.
CNNA's Amanda Watts has contributed to this report.