The infant's grandparents were baby-sitting on Saturday while the mother, who works the night shift, slept at home, Walker County Sheriff Steve Wilson said.
That was at around 3 pm, a time when the sweltering northwest Georgia heat hit 32 degree celsius.
In those conditions, Wilson said temperature in a closed vehicle can reach very high in as little as 15 minutes.
It was only when the boy's mother woke up and asked where her son was "that the grandparents realised that the child wasn't (inside)," CNN quoted Wilson as saying.
Despite attempts by the mother and then first responders to revive the child with Cardiopulmonary resuscitation (CPR), he was pronounced dead at the hospital.
No charges have been filed, but the death is being investigated by the Walker County Sheriff's Office, Wilson said.
The infant was the 19th child to die in the United States in 2015 because of being left inside a hot car, according to the Department of Meteorology & Climate Science at San Jose State University in California.
Widespread attention was brought to the issue in 2014 after a Georgia father spent an entire day at the office while his nearly 2-year-old son was still strapped inside his car seat.
Justin Ross Harris has pleaded not guilty to felony murder charges in the death of 22-month-old Cooper Harris.