De Marquise Elkins of Brunswick, who was 17 when he shot Antonio Santiago, 13 months, was found guilty of malice murder, felony murder and a long list of other charges, WXIA-TV in Atlanta reported. Jurors deliberated for only two hours after a 10-day trial before reaching their verdict.
His mother, Karimah Elkins, 36, was convicted of tampering with evidence and acquitted of lying to police.
Elkins does not face the death penalty because of his age at the time of the crime. He could receive a life sentence, while his mother faces a maximum of 10 years.
The trial was moved to Marietta because of the pre-trial publicity, but the sentencing will be held in Brunswick. No date has been set.
In his closing statement earlier Friday, Elkins' lawyer, Jonathan Lockwood, argued that police, under pressure to make an arrested in the case, fixed on his client.
"In this case the government, law enforcement, made no effort to look beyond the person they arrested," Lockwood said.