Justin Ross Harris, 35, had denied intentionally leaving his son, Cooper, to die in the car park near his Atlanta office in 2014.
Harris argued the death was an accident, saying he had forgotten to drop the toddler off at day care.
The trial heard he was sending lewd text messages to underage girls during the time that Cooper was dying.
The children left behind in hot cars
He was found guilty on eight counts, including malice murder as well as charges of child cruelty and sex crimes related to the text messages he sent to a teenage girl.
Prosecutors argued Harris plotted to kill his 22-month-old son and wanted to leave his family to continue affairs with other women.
But Harris' attorneys said that though he was responsible for the boy's death, he loved his son and the incident was result of a tragic forgetfulness.
Cobb County Police Detective Phil Stoddard had testified in court that Harris was sitting in his office, exchanging nude photos with several women, including a teenage minor, on the day his son died.
Mr Stoddard also told the grand jury panel Harris had researched online how long and what temperature it would take to die in a hot car.
He also said Harris looked at websites advocating a child-free lifestyle.
The child died of hyperthermia as his body overheated amid outside temperatures of almost 90F (32C).
Harris told authorities he had taken his son to breakfast and given him a kiss while putting him into the back of the car afterwards. But then he drove to work and forgot he was still in the car.
Harris also said it was a break from his usual routine. He typically ate after taking his son to nursery.
Car park surveillance footage showed Harris returned to his car after lunch to drop off some light bulbs he had bought.
He did not get inside the vehicle.
Friends and family testified on Harris' behalf, including his ex-wife, Leanna Taylor, who divorced him after the arrest.
She said while he "destroyed" her life, he was a loving father who would not intentionally harm their son.
Throughout the five-week trial, Harris cried when photos of his son were shown to the jury, according to Reuters news agency.
A man has been found guilty in the US state of Georgia of murdering his young son by leaving him in a hot car.