The toddler was fast asleep when his mother parked her silver Land Rover Freelander to deliver a package and left the engine running.
The horrified 32-year-old was at the door of a property handing over the parcel when she looked back and saw the suspect laugh at her, jump in the car and drive off from Kents Hill Road North, Benfleet, Essex, at 2.20pm on Friday.
The car was recovered shortly later in nearby Barncombe Close with the child still asleep.
The suspect fled and was not found despite Essex Police scrambling a search helicopter.
He was described as white, aged in his early 20s, clean shaven, and wore a black top with a light blue hood and dark trousers or tracksuit bottoms.