Corey Morgan reportedly showed no emotion when the verdict was announced. He faces up to 100 years in prison.
Tyshawn Lee was lured into a South Side alley with the promise of a juice box before being shot several times at close range, prosecutors said. The act was carried out in revenge, they said, as the men believed the fourth-grader's father was responsible for a shooting that left Morgan's brother dead and injured his mother.
A separate jury convicted co-defendant Dwright Boone-Doty of first-degree murder on Thursday; he could also be sentenced to up to 100 years in prison. A third man, Kevin Edwards, pleaded guilty in September in exchange for a 25-year prison term.
The homicide has been described as brutal and shocking, even for a city where gun violence occurs on an almost daily basis.
After the death of his brother, Morgan plotted to kill a relative of Tyshawn Lee's father, according to testimony.
Edwards' sister, Moesha Walker, testified that Morgan said "everybody must die" -- "Grandmas, mamas, kids and all."
"They went after his family, he's going after their family," Assistant State's Attorney Craig Engebretson told jurors to remind them of Morgan's quest for revenge.
The trio of men saw the boy playing basketball and struck up a conversation with him, prosecutors have said.
Then Boone-Doty coaxed Tyshawn into an alley by promising him a juice box before shooting him several times, according to prosecutors, who said Morgan gave Boone-Doty the gun that he used in the murder. The gun, they say, was purchased in New Mexico by one of Morgan's brothers and mailed to him.
Boone-Doty's DNA was found on the basketball that Tyshawn had been playing with. No evidence linked Morgan to the crime, but prosecutors called on witnesses who testified that they saw Morgan at the park where the murder took place and saw him put what appeared to be a gun into Boone-Doty's pocket.
Prosecutors also presented cellphone and geolocation evidence they said put Morgan at the park on the day of the crime. Morgan started looking at Tyshawn's parents' Facebook pages following the murder, prosecutors said.
"He wants to see them hurt the way he hurt," Engebretson said.
Morgan's lawyers claimed their client was being targeted because he was a gang member, and the person who identified him as being at the park that day only did it after asking about a reward.
Placing Morgan at the scene "made sense" because of the shooting of his brother and mother, attorney Todd Pugh told the jury on Thursday.
"He, in the eyes of [the] police, is one of those throwaway people," Pugh said, later adding: "He was a gangbanger who police thought was never going to amount to anything."
The Associated Press contributed to this report.