The child was one of seven toddlers between the ages of 2 to 4 left in the vehicle outside Waldorf's St. Charles Towne Center, according to the Charles County Sheriff's Office.
Authorities said the boy used a phone in the car to call for help as temperatures rose, WJLA reported.
While the boy didn't know where the vehicle was, officials were able to use GPS technology to locate it. Officers noted that the car was unlocked but had its windows up and was turned off when they arrived, according to WJLA.
The children appeared "scared" and "sweaty," WJLA reported. According to officials, the children were left alone for at least 20 minutes.
As officials were on the scene, the woman driving the vehicle came outside, but would not say why she had left the children in the car. She is the mother of two of the toddlers inside the vehicle.
The woman, who has not been identified, was taken into custody and charged with confinement in a motor vehicle.