Jurors were presented with evidence about the beating and what happened after Smith went to a neighbor's house for help.
The victim's family expressed joy after hearing the sentence.
"I feel happy that he's gone for life, but then that doesn't bring my son back," Dolisha Campbell told Eyewitness News.
On the day her son died, 24-year-old Campbell was at work. She left her son in Smith's care.
According to testimony provided during this week's trial, Smith hit 2-year-old Charvis Dublin, Jr. 41 times. The blows cause the child's brain to bleed and his liver to burst.
After the boy died, Smith took him to a neighbor's apartment so she could call 911.
Jurors learned Smith did not call 911 because he said he only had a few months left on his pre-paid cell phone.
While the neighbor called 911 and performed CPR, Smith watched a music video on TV.
The neighbor who called 911 testified at the trial. She said she tried to revive Dublin as Smith watched the music video and commented that he needed to purchase a CD.
Campbell said she still doesn't know why her former boyfriend hurt the her son.
"How [did] 41 bruises get on my son?" she asked. "What in the world did my little 2-year-old do to you for you to want to beat him?"
Smith took the stand in his own defense and told the court he was a member of the Bloods. He insinuated rival members of the Crips gang killed the child.
He said it might have happened when he left the boy to go deal marijuana.
Smith's attorney said it was a tough case and his client is now being vilified in Internet chat rooms.
"That doesn't surprise me," Attorney George Kelly said. "It was a tough case and a lot of negative evidence about the defendant."
Smith will serve 25 years to life in prison without parole.