Cellphone video WSB-TV obtained shows shoppers flagging down employees and calling 911 when they noticed the young boy asleep in the vehicle with its windows closed.
This happened as temperatures in metro Atlanta reached around 90 degrees, making it the hottest day of the year so far.
A witness who shared the video said the child looked to be about 2 years old and he was sweating.
Everyone helped to get water on the child while police waited for a parent to show up, which happened around 7:15 p.m.
A different video, taken later, shows police questioning the father, identified as Christopher Urgent, in the parking lot.
Witnesses said the man was inside the store with two other children.
McDonough police eventually arrested Urgent on a cruelty to children charge.
"There ain't no excuse for leaving no kid in no car, especially in the heat out here," shopper Noel Echevarria said.
The young boy was taken to a hospital by ambulance to be checked out further, but witnesses said he seemed to be OK.